Sensors, Pixels and Image Sizes - Photocourse.com

AA30470C

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An Extension to PhotoCourse Digital Photography Textbooks

Sensors, Pixels and Image Sizes AA30470C

Dennis P. Curtin http

: / / w w w . ShortCourses. c o m

h t t p : ,/visit / whttp w :// wwww . P.shortcourses h o t o C o.com urse.com For more on digital photography

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Sensors, Pixels and Image Sizes

ShortCourses

and

PhotoCourse Publishing Programs

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hort Courses and its sister site PhotoCourse.com, are the leading publishers of digital photography books, textbooks, and easy to follow guides to specific cameras.

Classroom Discounts PhotoCourse and ShortCourses books are used in hundreds of schools, adult and community education programs, and in major camera company and government departments. If you are an instructor, special pricing is available for classroom use. • For details on using our texts in the classroom, visit our textbook Web site at www.photocourse.com or call us at 781-631-8520, Boston, Massachusetts USA time. • For camera guides and other digital photography books, visit the Short Courses bookstore at www.shortcourses.com/bookstore/book.htm. If you find any errors in this book, would like to make suggestions for improvements, or just want to let me know what you think—I welcome your feedback, even though I’m not always able to respond personally. Contact/Feedback Information ShortCourses.com 16 Preston Beach Road Marblehead, Massachusetts 01945 E-mail: [email protected]

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Contents

Contents Cover ShortCourses and PhotoCourse Publishing Programs Contents Introduction Image Sensors—Introduction Types of Image Sensors Image Sensors and Pixels Image Sensors and Image Size Exploring Images Sizes Image Sensor Sensitivity and Noise Image Sensor Sizes Aspect ratios Exploring Aspect Ratios Image Sensors and Color Cleaning Image Sensors Pixels and Screen Display Exploring Pixels on the Screen Pixels and Print Sizes Exploring Print Sizes Pixels per Inch Exploring Pixels Per Inch Pixels and Colors Exploring Pixels and Colors Scanning and Image Sizes The Complete Excel Worksheet

List

of

1 2 3 4 5 6 7 9 13 14 15 16 18 19 22 23 25 26 27 28 30 31 32 33 37

Animations & Extensions

Visit ShortCourses.com Web Site Visit PhotoCourse.com Web Site Understanding Exposure Where “Charge-Coupled” Comes From Dots On An Ink-Jet Print Pixels And Curves Pixelization Output Devices Determine Image Sizes Resolution—The Original Meaning Calculating Image Sizes Image Sensor Sizes Calculating Aspect Ratios ISO and Noise Noise, Example From Cadillac Ranch Rgb Color Dust On Your Image Sensor Exploring Pixels on the Screen Exploring Print Sizes Exploring Pixels per Inch Exploring Pixels and Colors Scanning and Image Sizes

2 2 5 6 7 9 10 11 12 13 15 18 14 14 19 22 25 27 30 32 33

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Introduction Kodak is one of the world’s largest sensor manufacturers. Here is one of their latest silicon waffers containing a number of image sensors. Courtesy of Kodak (www.koday. com)

The 2009 Nobel Prize in Physics was awarded to Willard S. Boyle and George E. Smith for inventing the CCD (charge-coupled device) in 1969 at Bell Labs in New Jersey. Charles K. Kao also shared the prize for his work in fiber optic light transmission.

D

igital images are formed from tiny dots of color. The dots, usually many millions per image, are so small and close together they blend into the smooth continuous tones we’re so familiar with from film. These images are captured directly with digital cameras, or by scanning a transparency, negative, or print. The end result is an image in a universally recognized format that can be easily manipulated, distributed, and used. However, just because photography is an art form, it doesn’t mean you don’t have to know some math. When it comes to displaying or printing your images, this section could be titled “So You Have to Know Arithmetic After All.” If your camera captures an image that’s 3648 x 2736 pixels in size, what does that mean when you e-mail it, post it to a Web site, or make a print? In this extension you find out how to answer these kinds of questions. The math we’ll be using in this extension is limited to simple subtraction, addition, multiplication and division—subjects you mastered early in school. However, to make it easier to explore the various relationships being discussed, you can use the Excel worksheet “Pixels & Images Calculator” downloadable at the link below or by clicking the Excel button in this extension: http://www.photocourse.com/itext/pixels/pixelcalc.xls The worksheet, named pixelcalc.xls, has been saved in Excel 5 format so that version and all later versions of Excel can read it. 4


Image Sensors—Introduction

Image Sensors—Introduction Digital cameras have roots going back almost 200 years. Beginning with the very first camera all have been basically black boxes with a lens to focus the image, an aperture that determines how bright the light is that enters the camera, and a shutter that determines how long the light enters. The big difference between traditional film cameras and digital cameras is how they capture the image. Instead of film, digital cameras use a solid-state device called an image sensor. On the surface of these fingernail-sized silicon chips are millions of photosensitive diodes, called photosites, each of which captures a single pixel in the photograph to be. An image sensor sits against a background enlargement of its square pixels, each capable of capturing one pixel in the final image. Courtesy of IBM.

A CCD is like a threedecker sandwich. The bottom layer contains the photosites. Above them is a layer of colored filters that determines which color each site records. Finally, the top layer contains microlenses that gather light for each photosite. Courtesy of Fujifilm.

When you take a picture, the camera’s shutter opens briefly and each pho-

tosite on the image sensor records the brightness of the light that falls on http://www.photocourse.com/itext/exposure/ Click to explore how exposure determines how light or dark an image is.

it by accumulating photons. The more light that hits a photosite, the more photons it records and the brighter it will be. Photosites capturing light from highlights in the scene will have many photons. Those capturing light from shadows will have few. After the shutter closes to end the exposure, the photons from each photosite are counted and converted into a digital number. This number represents the brightness of a single pixel and it, along with pixels captured by all of the other photosites on the sensor, is used to reconstruct the image by setting the brightness and color of matching pixels on the screen or printed page.


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Types

of

Image Sensors

This photo shows the pixels on an image sensor greatly enlarged. Courtesy of IBM.

A silicon wafer used to make image sensors. Courtesy of IBM.

When using a film camera you can insert any kind of film you want. It’s the film you choose that gives photographs distinctive colors, tones, and grain. If you think one film gives images that are too blue or red, you can change to another film although choices are growing more limited. With digital cameras, the “film” is permanently part of the camera so buying a digital camera is in part like selecting a film to use. Like film, different image sensors and the software used to process the images it captures render colors differently, have different amounts of “noise,” different sensitivities to light, and so on. The only ways to evaluate these aspects are to examine some sample photographs from the camera or read reviews written by people you trust. Initially, charge-coupled devices (CCDs) were the only image sensors used in digital cameras. They had already been well developed through their use in astronomical telescopes, scanners, and video camcorders. However, there is also a well-established alternative, the CMOS image sensor. Both CCD and CMOS image sensors capture light using a grid of small photosites on their surfaces. It’s how they process the image and how they are manufactured where they differ from one another. • CCD image sensors. A charge-coupled device (CCD) gets its name from the way the charges on its pixels are read after an exposure. The charges on the first row of the sensor are transferred to a circuit called the read out register. From there, they are fed to an amplifier and then on to an analogto-digital converter. Once a row has been read, its charges in the readout register row are deleted, the next row enters, and all of the rows above march down one row. With each row “coupled” to the row above in this way, each row of pixels is read—one row at a time.

• CMOS image sensors. Image sensors are manufactured in factories called wafer foundries or fabs where the tiny circuits and devices are etched onto silicon chips. The biggest problem with CCDs is that they are created in http://www.photocourse.com/itext/CCD/ foundries using specialized and expensive processes that can only be used to Click to see where the make other CCDs. Meanwhile, larger foundries use a different process called name “charge-coupled Complementary Metal Oxide Semiconductor (CMOS) to make millions of device” comes from. chips for computer processors and memory. CMOS is by far the most common and highest yielding chip-making process in the world. Using this same process and the same equipment to manufacturer CMOS image sensors cuts costs dramatically because the fixed costs of the plant are spread over a much larger number of different kinds of devices. As a result of these economies of scale, the cost of fabricating a CMOS wafer is significantly less than the cost of fabricating a similar wafer using the specialized CCD process. Costs are lowered even farther because CMOS image sensors can have processing circuits created on the same chip. With CCDs, these processing circuits must be on separate chips. Despite their differences, both types of sensors are capable of giving very good results and both types are used by major camera companies. Canon and Nikon both use CMOS sensors in their high-end digital SLRs as do many other camera companies.

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Image Sensors and Pixels

Image Sensors

and

Pixels

Digital photographs are actually mosaics of millions of tiny squares called http://www.photocourse.com/itext/dots/ Click to see how pixels are printed using dots of colored ink.

picture elements—or just pixels. Like the impressionist painters who painted wonderful scenes with small dabs of paint, your computer and printer use these tiny pixels to display or print photographs. To do so, the computer divides the screen or printed page into a grid of pixels much as the sensor is divided into a grid of photosites. It then uses the values stored in the digital photograph to specify the brightness and color of each corresponding pixel in this grid—a form of painting by number.

This reproduction of the famous painting “The Spirit of ‘76” is done in jelly beans. Think of each jelly bean as a pixel and it’s easy to see how dots or pixels can form images. Jelly Bean Spirit of ’76 courtesy of Herman Goelitz Candy Company, Inc. Makers of Jelly Belly jelly beans.

Now that you understand a little about pixels and images, let’s introduce one surprising fact: A pixel has no size or shape. It begins its life on the camera’s image sensor during that flickering moment when the shutter is open. The size of each photosite on the image sensor can be measured, but the pixels themselves are just photons, soon to be converted into electrical charges, and then into zeros and ones. These numbers, just like any other numbers that run through your head, have no physical size. A pixel is only given size and shape by the device you use to display or print it. Understanding how pixels and image sizes relate to one another takes a little effort but you need to bring nothing more to the process than your curiosity and elementary school arithmetic skills.


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Sensors, Pixels and Image Sizes Although the captured pixels have no physical dimensions, a sensor’s size is specified just like a digital photo’s, except the count is the number of photosites that it has on its surface instead of pixels. In most cases the numbers of photosites and the number of pixels are roughly the same since each photosite captures one pixel. (The numbers don’t match exactly because some sensor photosites are used for purposes other than capturing images.) The two are so closely related, many people refer to both as pixels. Perhaps the largest image sensor to date is the LSST (large Synoptic Survey Telescope) array. It's diameter, like the mockup shown here, is 25 inches (64 cm) and it captures over 3 Gigapixels per image. (Image credit: LSST Corporation)

Since pixels stored in an image file have no physical size or shape, it’s not surprising that the number of pixels doesn’t by itself indicate a captured image’s sharpness or size. This is because the size of each captured pixel, and the image of which it’s a part, is determined by the output device. The device can spread the available pixels over a small or large area on the screen or printout. If the pixels in an image are squeezed into a smaller area, the image gets smaller and the perceived sharpness increases (from the same viewing distance). Images on high-resolution screens and printouts look sharper only because the available pixels are smaller and grouped into a small area—not because there are more pixels. As pixels are enlarged, an image is spread over a larger area, and its perceived sharpness falls again (from the same viewing distance). When enlarged past a certain point, the individual pixels begin to show—the image becomes pixilated. To visualize this concept, imagine two tile mosaics, one with small tiles and one with large.

Canon makes a 120 Megapixel CMOS sensor that can capture 9.5 frames per second and record Full HD movies.

• If both mosaics cover an area of the same size, the one created using small tiles has more tiles so it has sharper curves and more detail. • If there are the same number of large and small tiles, the area covered by the small tiles is smaller. When viewing both mosaics from the same distance, the smaller one looks sharper. However, if you view the small mosaic from close up, its sharpness and detail can be made to appear identical to the larger one viewed from farther away. To make an existing image larger or smaller for a given output device, you must add or subtract pixels. This process, called resampling, can be done with a photo-editing program, or by an application you’re using to print an image. • When an image is resampled to make it larger, extra pixels are added and the color of each new pixel is determined by the colors of its neighbors. The problem with this is that it can’t be done without degrading image quality. • When an image is resampled to make it smaller, some pixels are deleted and image quality is otherwise retained.

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Image Sensors and Image Size

Image Sensors

and

Square pixels are arranged in patterns to form curved lines and edges in a photo. The more pixels used, the smoother these curves will be. Here the same red ball is represented by 4, 12, and then 24 square pixels. As more pixels are added, edges become more refined and the shape becomes more like the original.

Image Size

When capturing an image, the number of pixels used to capture it (sometimes referred to as resolution or pixel count) has a big effect on how large it can be displayed on the screen or printed. At any given size, more pixels add detail and sharpen edges. Because numbers matter so much, the best approach is to shoot using the largest available size. You can always make an image smaller in a photo-editing program, but you can never make it larger while retaining the original quality.

The pixel size of a digital image is specified in one of two ways—by its dimensions in pixels or by the total number of pixels it contains. For example, the same image can be said to have 4368 × 2912 pixels (where “×” is pronounced “by” as in “4368 by 2912), or to contain 12.7 million pixels (4368 multiplied by 2912). Since the term “megapixel” is used to indicate 1 million pixels, an image with 12 million pixels can also be referred to as a 12 megapixel image.

Image sizes are expressed as dimensions in pixels (4368 × 2912) or by the total number of pixels (12,719,616).

No matter how many pixels an image has, when you enlarge it enough, it

http://www.photocourse.com/itext/pixelresolution/ begins to loose sharpness and eventually the pixels begin to show—an effect Click to explore how more pixels give sharper images.

called pixelization. This is not unlike traditional silver-based prints where grain begins to show when prints are enlarged past a certain point. The more pixels an image has, the larger it can be displayed or printed before pixelization occurs. However, with even inexpensive cameras capturing 6 and 8 megapixel images, most images will never bump up against this limit even when enlarged to 8 x 10 inches or more. Another advantage of larger images is seen when editing. Not only can you crop them more, changes to such aspects as color balance, hue, saturation,


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Sensors, Pixels and Image Sizes When a digital image is displayed or printed at the correct size for the number of pixels it contains (left), it looks like a normal photograph. When enlarged too much (right), its square pixels begin to show.

http://www.photocourse.com/itext/pixelzoom/ Click to see the effects of pixelization as an image is enlarged.

contrast, and brightness are more effective on larger images because there is more image data to work with and tonal gradations are much smoother. After making these adjustments, you can reduce the file to the needed size. As you might expect, all else being equal, costs rise as the size of the image sensor increases. Although larger sensors can give you sharper images and better enlargements, more pixels also means larger image files. Not only do larger files take up more storage space, they take longer to transfer, process, and edit and are often far too large to e-mail or post on a Web site. Smaller image sizes such as 800 x 600 are perfect for Web publishing, e-mail attachments, small prints, or as illustrations in your own documents and presentations. For these uses, higher resolutions just increase file sizes without significantly improving the images. Choosing image sizes The camera you use determines how large your images can be, but most also allow you to select smaller sizes. Here are some rules of thumb about what image sizes you need for certain outputs. • On the Internet, images are displayed on screens that have resolutions of 1280 x 1024, 1152 x 864, 1024 x 768, 800 x 600, or 640 x 480. A few years ago, a 1024 x 768 monitor was unusual so most people in the industry settled on assuming that the lowest common denominator for screen sizes was 640 x 480 or, at best 800 x 600. For this reason, images to be e-mailed or posted on the Internet should be of similar or smaller sizes—no more than 800 pixels wide. This ensures that the images will display correctly on the vast majority of computers. If an image is too large, users will not be able to see it all at once and will be forced to scroll around it. If too small, details will be lost. Size also affects the speed with which images travel over the Web. Smaller (and more compressed) images travel faster so people see them more quickly.

One advantage of a large image size is that it gives you the freedom to crop the image and still have it be a usable size.

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• For laser and inkjet printers you need between 200–300 image pixels per inch. If your camera can capture images that are 2400 pixels wide, you can expect good results when prints are up to 12 inches wide. • When images are printed on a printing press, as they might be for a catalog, the pixels in the image are printed as dots on the page. Photographs For more on textbooks in digital photography, visit http://www.photocourse.com

Image Sensors and Image Size that are to be printed on a press are first digitally “screened” to break the image up into dots. If you are ever involved in this process your print shop will give you specifications for your images The number of pixels in an image, sometimes referred to as its resolution, determines the size of the image when it’s displayed on the screen or how large a print can be made that is still sharp.

SCREEN RESOLUTIONS CGA EGA VGA SVGA XGA SXGA WXGA SXGA+ UXGA WSXGA+ WUXGA QXGA QSXGA QUXGA WQUXGA

320 x 200 640 x 350 640 x 480 800 x 600 1024 x 768 1280 x 1024 1366 x 768 1400 x 1050 1600 x 1200 1680 x 1050 1920 x 1200 2048 x 1536 2560 x 2048 3200 x 2400 3840 x 2400

http://www.photocourse.com/itext/imagesize/ Click to see how it’s the output device, not the camera, that determines image sizes.

Here are the relative sizes of two images sized to be printed or displayed at 5 x 4 inches. The larger image (1500 x 1200 pixels) will print at 300 dots per inch. The smaller one (360 x 288) will be displayed on the screen at 72 dpi. Although greatly different in the number of pixels they contain, the different output devices will render them the same size.


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Sensors, Pixels and Image Sizes Interpolated

If an image is too large for a screen (top), the viewer has to scroll around it. When sized correctly (bottom) they can see the entire image. Most digital photography programs automatically resize images to fit the available screen resolution unless you specify a different size.

resolution

Beware of claims about resolution for cameras because there are two kinds of resolution; optical and interpolated. The optical resolution of a camera is an absolute number because an image sensor’s pixels or photosites are physical devices that can be counted. However, optical resolution can be increased using a process called interpolated resolution that adds pixels to the image to increase the image’s size. To do so, software evaluates those pixels surrounding each new pixel to determine what its color should be. For example, if all of the pixels around a newly inserted pixel are red, the new pixel will be red. What’s important to keep in mind is that interpolated resolution doesn’t add any new information to the image—it just adds pixels and makes the file larger. This same thing can be done in a photo-editing program such as Photoshop by resizing the image. Beware of companies that promote or emphasize their device’s interpolated (or enhanced) resolution. You’re getting less than you think you are. Always check for the device’s optical resolution. If this isn’t provided, you’re dealing with marketing people who don’t have your best interests at heart. One term—two meanings The term “resolution” has two meanings in photography. Originally it referred to the ability of a camera system to resolve pairs of fine lines such as those on a test chart. In this usage it’s an indicator of sharpness, not image size. With the introduction of digital cameras the term began being used to indicate the number of pixels a camera could capture. Two meanings for the same term is not a good turn of events in any field.

http://www.photocourse.com/itext/resolution/ Click to explore the original meaning of “resolution.”

Test charts have pairs of lines at various spacings

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Exploring Images Sizes

Exploring Images Sizes Part 1 on the Excel worksheet “Pixels & Images Calculator” calculates the http://www.photocourse.com/itext/pixels/pixelcalc.xls Click to open an Excel worksheet and use Part 1. Image Sizes to follow the instructions in this section.

total number of pixels in an image and its aspect ratio when you enter the image’s width and height in pixels. The numbers in the descriptions that follow refer to row numbers on the worksheet.

1. Width of image (in pixels) is where you enter the image’s width in pixels. 2. Height of image (in pixels) is where you enter the image’s height in pixels. 3. Total number of pixels in image is calculated by multiplying the image’s width by its height. 4. Aspect ratio is discussed in the next section. Exercises Open the PixelCalc.xls worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the following questions. 1. Enter the width and height of your own images to find the total number of pixels they contain and their aspect ratio. You can find image sizes in your camera’s user guide. There may be more than one resolution, and if so, try them all. When the resolution changes, does the aspect ratio also change? 2. If a digital camera recorded the following image sizes, how many pixels are there in each image size? What is the aspect ratio for each? • 2,400 x 1,800 ____________________________ • 1280 x 960 ______________________________ • 640 x 480 _ _____________________________ 3. Here are some rectangles commonly found in photography. Calculate the total number of pixels (in the images). Typical Image Sizes Image Computer display

Width x Height

Pixels

Aspect Ratio

1024 x 768

.

Canon 5D

4368 x 2912

.

Canon S3 IS

2816 x 2112

.


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Image Sensor Sensitivity TIP To see one kind of noise, leave the lens cap on and take a picture. The long exposure will create noise that you can see when you open the picture in a photo editing program and enlarge it.

and

Noise

In some situations images are not as clear as they might otherwise be. They appear grainy with randomly scattered colored pixels that break up smooth areas. This is what’s known as noise. It has three basic causes: • Small photosites on the sensor. There is nothing you can do about this cause, but it also makes the following causes even more severe. • A long shutter speed that lets light into the camera for a long time, usually in a dim or dark setting, gives noise a chance to build up. • A high ISO setting let’s you use a faster shutter speed to avoid blur but also amplifies the noise along with the signal. Many cameras have one or more noise reduction modes that reduce the effects of this noise.

Noise appears in images as random color pixels especially when you use long shutter speeds or high ISO settings.

http://www.photocourse.com/itext/ISO/ Click to see the effects of increasing ISO.

http://www.photocourse.com/itext/noise/ Click to see the effect of noise in an image.

At slow shutter speeds (left) the exposure, like dripping water, is so slow noise has a chance to build up in the image. At faster speeds, (middle and right), the noise is overwhelmed by how fast the exposure is completed.

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Image Sensor Sizes

Image Sensor Sizes Image sensors come in a variety of sizes with the smallest ones used in point and shoot cameras and the largest in professional SLRs. Consumer SLRs often use sensors having the same size as a frame of APS film. Professional SLR cameras occasionally use sensors the same size as a frame of 35mm film— called full-frame sensors. (Large format cameras use even larger sensors.) Image sensor sizes range from the tiny up to ones as large as a frame of 35mm film—called a full frame sensor.

http://www.photocourse.com/itext/sensor/ Click to explore the sizes of image sensors.

Larger image sensors generally have larger photosites that capture more light with less noise and a greater range of tones for smoother transistions. The result is pictures that are clearer, brighter, and sharper. Because the size of photosites is so important, a large 8 Megapixel sensor will often take better pictures than a smaller 12 Megapixel sensor. Not only is noise a problem but smaller sensors also require better, more expensive lenses, especially for wide-angle coverage. Here are some typical sensor sizes: Image sensors in camera phones are very small. Courtesy of OmniVision.

TIP A 16:9 wide-screen mode captures images and film clips that are perfect for display on your widescreen TV or computer monitor.

Size

Width (mm)

Height (mm)

Used in

1/2.5

5.76

4.29

Point & Shoot Cameras

1/q.8

7.18

5.32

Point & Shoot Cameras Point & Shoot Cameras

1/1.7

7.6

5.7

APS-C

22.2

14.8

Consumer SLRs

4/3 system

23.6

15.7

Four-thirds System Cameras

APS-H

28.7

19

Consumer SLRs

Full frame

36

24

Professional SLRs

APS-C sensors listed in the table above are based on the size of APS-C film (25.1 x 16.7 mm ), but come in a variety of slightly different sizes. For example, Canon uses sensors that are 22.2 x 14.8 mm or 28.7 x 19.1 mm, Sony’s are 21.5 x 14.4 mm, and Nikon’s (called “DX” sensors), are 23.7 x 15.7 mm.


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Aspect

ratios

Image sensors come in a variety of aspect ratios—the ratio of the sensor’s width to height. The ratio of a square is 1:1 (equal width and height) and that of 35mm film is 1.5:1 (1½ times wider than it is high). Most image sensors fall in between these extremes. The aspect ratio of a sensor is important because it determines the shape and proportions of the photographs you create. When an image has a different aspect ratio than the device it’s displayed or printed on, it has to be cropped or resized to fit. Your choice is to crop part of the image or waste part of the paper or display area. To imagine this better, try printing a square image on a rectangular sheet of paper so either the entire image is printed or the entire paper is filled. Image

Width x Height

Aspect Ratio

36 x 24

1.50

Computer display

1024 x 768

1.33

Canon 5D

4368 x 2912

1.50

Canon S3 IS

2816 x 2112

1.33

Photo paper

4x6

1.50

8.5 x 11

1.29

16 x 9

1.80

35mm film

Printing paper HDTV

To calculate the aspect ratio of any camera, divide the largest number in its resolution by the smallest number. For example, if a sensor has a resolution of 4368 x 2912, divide the former by the later. In this case the aspect ratio is 1.5, the same as 35 mm film but different from an 8.5 x 11 sheet of paper.

The aspect ratio of an image sensor determines the shape of your prints.

TIP Digital SLRs generally have an aspect ration of 3:2 (1.5:1), the same aspect ratio as 35mm film.

If you have ever tried to center a photo on a sheet of paper so there are even borders all around the image, you have been dealing with the concept called “aspect ratios.” This is the ratio between the width and height of an image, screen display, paper or any other two dimensional rectangle. To calculate an aspect ratio, divide the largest number in a rectangle’s size by the smallest number. The numbers can be mm, inches, pixels or any other unit of measurement. For example, a 35 mm slide or negative is 1.5 inches wide by 1 inch tall so its aspect ratio is 1.5 to 1. A square has an aspect ratio of 1:1. If a camera captures images 3000 x 2000 pixels in size, 3000 divided by 2000 gives an aspect ratio is 1.5, the same as 35mm film. Aspect ratios are usually expressed in one of three ways: • When expressed as 1.5 to 1 or 1.5:1, the actual numbers calculated in the division process are used, even though one has a decimal place. • To remove the decimal, the numbers are raised to a new ratio so both numbers are even. In our example, 1.5 to 1 would be raised to 3 to 2. That’s what’s done with TV screen aspect ratios. The aspect ratio for normal TV is referred to as 4:3 and HDTV as 16:9. • In a few cases, where one part of the ratio is assumed to be 1, just the other part is given. For example, a 1.5:1 ratio is expressed as 1.5. Aspect ratios present a problem when printing or displaying images. Most cameras don’t capture images with the same aspect ratio as the 11 x 8.5 paper we print on—which has an aspect ratio of 1.29 (11 divided by 8.5). Few have the same aspect ratio as the screens we display them on. Even some software that prints contact sheets crops images—greatly lowering their usefulness when you try to evaluate images for printing. When the aspect ratios don’t match, here are your options:

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Aspect ratios • Crop the image to the desired aspect ratio. Programs such as Photoshop let you crop or select areas of an image using any aspect ratio that you specify. To do it manually you: 1. Determine the aspect ratio you want to use. 2. Determine how high the image needs to be in pixels. 3. Multiply the height by the aspect ratio to determine how wide the image should be in pixels. When you know the width and want to find the height, divide the aspect ratio’s largest number into the smaller and multiply the width by that number. For example if the aspect ratio is 1.5:1 divide 1 by 1.5 to get 0.667. If the image is 3 inches wide, 3 x 0.667 tells you the height is 2 inches. • Size the image so it fills the available space in one direction even though some of it extends past the edges in the other dimension. In effect, you are cropping the image. • Size the image leaving unequal borders around it. You can then trim it or fix the problem while matting it for framing. These examples illustrate different aspect ratios. The top image was sized to fill the width of a sheet of 11 x 8.5 paper. This leaves empty bands at the top and bottom of the paper. The bottom image was sized to fill the height of the paper but parts of the image extending past the sides are left unprinted.

Images sensors come in a variety of sizes and aspect ratios. At the time this was being written, 60 megapixels was the largest image sensor used in digiatl cameras. Courtesy of Dalsa


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Exploring Aspect Ratios Part 1 on the Excel worksheet “Pixels & Images Calculator” calculates the total number of pixels in an image and its aspect ratio when you enter the image’s width and height in pixels. The numbers in the descriptions that follow refer to row numbers on the worksheet.

http://www.photocourse.com/itext/pixels/pixelcalc.xls Click to open an Excel worksheet and use Part 1. Image Sizes to follow the instructions in this section.

4. Aspect ratio is calculated by dividing the image’s width by its height. Exercises Open the PixelCalc.xls worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the following questions. 1. Enter the width and height of your own images to find the total number of pixels they contain and their aspect ratio. You can find image sizes in your camera’s user guide. There may be more than one resolution, and if so, try them all. When the resolution changes, does the aspect ratio also change? 2. If a digital camera recorded the following image size what is the aspect ratio for each? • 2,400 x 1,800 ____________________________ • 1280 x 960 ______________________________ • 640 x 480 _ _____________________________ 3. Here are some rectangles commonly found in photography. Calculate the total number of pixels (in the images) and the aspect ratios. Typical Aspect Ratios Image 35mm film

Width x Height

Aspect Ratio

36 x 24

.

1024 x 768

.

Canon 5D

4368 x 2912

.

Canon S3 IS

2816 x 2112

.

Computer display

Photo paper Printing paper HDTV

4x6

.

8.5 x 11

.

16 x 9

.

18


Image Sensors and Color

Image Sensors

and

Color

It may be surprising, but pixels on an image sensor only capture brightness, not color. They record the gray scale—a series of tones ranging from pure white to pure black. How the camera creates a color image from the brightness recorded by each pixel is an interesting story with its roots in the distant past. The gray scale, seen best in black and white photos, contains a range of tones from pure black to pure white.

RGB uses additive colors. When all three are mixed in equal amounts they form white. When red and green overlap they form yellow, and so on.

Maxwell (top) and his actual photograph of the tartan ribbon taken in 1861 (bottom).

When photography was first invented in the 1840s, it could only record black and white images. The search for a color process was long and arduous, and a lot of hand coloring went on in the interim (causing one photographer to comment “So you have to know how to paint after all!”). One major breakthrough was James Clerk Maxwell’s 1860 discovery that color photographs could be created using black and white film and red, green and blue filters. He had the photographer Thomas Sutton take three photos of a tartan ribbon, each time with a different color filter over the lens. The three black and white images were then projected onto a screen with three different projectors, each equipped with the same color filter used to take the image being projected. When brought into alignment, the three projected images formed a full-color photograph. Over a century later, image sensors work much the same way. Colors in a photographic image are usually based on the three primary colors red, green, and blue (RGB). This is called the additive color system because colors are created by mixing the three colors. This RGB system is used whenever light is projected to form colors as it is on the display monitor (or in your eye). Another color system uses cyan, magenta, yellow and black (CMYK) to create colors. This system is used in almost all printers since it’s the color system used with reflected light. It’s called subtractive because it absorbs, or subtracts, colors so only red, green, and blue are reflected. Since daylight is made up of red, green, and blue light; placing red, green,

and blue filters over individual pixels on the image sensor can create color http://www.photocourse.com/itext/RGB/ Click to explore how red, green and blue can create full color images.

images just as they did for Maxwell in 1860. Using a process called interpolation, the camera computes the actual color of each pixel by combining the color it captured directly through its own filter with the other two colors captured by the pixels around it.


19

Sensors, Pixels and Image Sizes Because each pixel on the sensor has a color filter that only lets through one color, a captured image records the brightness of the red, green, and blue pixels separately. (There are usually twice as many photosites with green filters because the human eye is more sensitive to that color so green color accuracy is more important.) Illustration courtesy of Foveon at www.foveon. com.

Each pixel on an image sensor has red, green, and blue filters intermingled across the photosites in patterns designed to yield sharper images and truer colors. The patterns vary but the most popular is the Bayer mosaic pattern shown here.

To create a full color image, the camera’s image processor calculates, or interpolates, the actual color of each pixel by looking at the brightness of the colors recorded by it and others around it. Here the full-color of some green pixels are about to be interpolated from the colors of the eight pixels surrounding them.

20


Image Sensors and Color There are at least 256 tones captured for each color—red, green, and blue. Only one tone at the shadow (black) end of the range and one at the highlight (white) end are pure and have no detail.

Each time you take a picture, millions of calculations are made in just a few seconds. It’s these calculations that make it possible for the camera to interpolate, preview, capture, compress, filter, store, transfer, and display the image. All of these calculations are performed in the camera by an image processor that’s similar to the one in your desktop computer, but dedicated to this single task. How well your processor performs its functions is critical to the quality of your images but it’s hard to evaluate advertising claims about these devices. To most of us these processors are mysterious black boxes about which advertisers can say anything they want. The proof is in the pictures.

Cameras with the latest programmable image processors can be programmed by camera companies to perform a variety of functions. Currently these functions include in-camera photo editing and special effects such as red-eye removal, image enhancement, picture borders, stitching together panoramas, removing blur caused by camera shake, and much more. When a camera company programs its processors its goal isn’t to exactly reproduce a scene’s colors. Instead, using a process called color rendering, its goal is to create what the programmers believe will be a pleasing reproduction. Frequently the contrast and color saturation are boosted, especially in the midtones and specular highlights are compressed for printing and viewing on typical displays. The processed images can be so distinctive that it’s possible for some people to tell when an image was taken with a Canon or Nikon camera.


21


Cleaning Image Sensors When you change lenses a lot on a digital SLR, or even once in a windy or http://www.photocourse.com/itext/dust/ Click to see the effects of dust on your images.

dusty environment, dust can enter the camera and stick to the low-pass filter covering the image sensor. This dust creates dark spots on any images you then capture. One way to check if this has happened is to take a few photos of a clear sky or white card. Open the images in your photo-editing program and enlarge them to see if there are any dark dust spots in what should be even, light areas. The dust problem is so serious that camera companies are doing everything they can think of to reduce it including the following: • Reduce the dust by minimizing the dust and particles created by the camera itself, by using materials in the body cap and shutter that don’t create dust and other particles during normal wear and tear. • Make it difficult for the dust to stick by coating the low-pass filter with a non-stick coating. (The low pass filter in front of the image sensor is designed to eliminate moiré patterns and give more accurate color.) • Repel the dust by applying an anti-static charge to the low pass filter covering the sensor to prevent static-charged dust from adhering to it. • Remove the dust by attaching an ultrasonic vibrating unit to the low-pass filter so it can shake off any dust particles that are adhering to it. The newly liberated dust is then captured by an adhesive material that prevents it from becoming airborne again. This shaking may occur automatically when you turn the camera on or off, or manually when you make a menu selection. • Put the dust out of focus. The low-pass filter, normally a single unit, can be divided into two layers, a front and a rear. The front layer, where any dust would accumulate, is positioned far enough away from the sensor so any dust on it will be out of focus and less likely to show in the images. • Process the dust away. You just photograph a white wall or sheet of paper (or, in a pinch, remove the lens from the camera) and the camera maps (records) the size and position of the dust particles on the low pass filter. This map is then attached to all images as metadata. When the images and appended dust data map are transferred to a computer, software supplied with the camera can use the information in the map to remove the effects of dust on the image.

Here are the five steps recommended by Photographic Solutions for cleaning your image sensor with their sensor swabs and Eclipse cleaning fluid. Courtesy of photosol.com.

22

• Manually clean the sensor. When all else fails your remaining choice is to return the camera to the camera company’s service center (tiresome after awhile) or clean it yourself (a high risk procedure). If you decide to do it yourself, you use a menu command that locks the mirror up and out of your way and opens the shutter so you can get to the surface of the image sensor. You then clean the sensor (actually the low pass filter) with sensor swabs and cleaning fluid developed specifically for this purpose. NEVER use compressed air, or other cleaning products, on the sensor. Cleaning supplies are available from sources such as B&H and Calumet. For more information Google “cleaning image sensor” but proceed at your own risk.


Pixels and Screen Display

Pixels

and

Screen Display When a digital image is displayed on the computer screen, its size is determined by three factors—the screen’s resolution setting, the screen’s size, and the number of pixels in the image. The screen’s resolution The size of each pixel on the screen is determined by the screen’s resolution setting. The resolution is almost always given as a pair of numbers that indicate the screen setting’s width and height in pixels. For example, a monitor may be specified as being a low-resolution 640 x 480, a medium resolution of 800 x 600, or a high-resolution of 1024 x 768 or more. (The first number in the pair is the number of pixels across the screen. The second number is the number of rows of pixels down the screen.)

This is a 1680 x 1050 display. That means there are 1680 pixels on each row and there are 1050 rows of pixels.

Screen resolution and image size On any given monitor, changing screen resolution changes the number and size of pixels used to display objects such as icons, text, buttons, and images. As shown in the margin illustrations, as the resolution increases, pixels and object sizes decrease making the objects appear sharper.

At 800 x 600 (top), Photoshop and the image being edited fills the screen. When the screen resolution is increased to 1024 x 786 (middle), the image is smaller and at 1280 x 1024 (bottom) even smaller.

One way to think about the size of each pixel is in terms of how many pixels are displayed per inch on the screen—pixels per inch (ppi). The larger the pixels, the fewer fit per inch. As you can see from the table on the facing page, the actual number of pixels per inch (the numbers in italics) depends on both the resolution setting and the size of the monitor. (Advertised screen sizes are based on a diagonal measurement. The sizes we’re referring to here are horizontal measurements across the screen so they don’t relate exactly to advertised screen sizes.) If a 14” monitor and a 21” monitor are both set to 800 x 600 pixels, the pixels per inch are different. On the larger screen the same 800 pixels are spread along a longer row so the pixels per inch decreases. One number you will often see quoted is 72 ppi. This is supposed to be a magic number in digital imaging. Its origins are said to go back to early Apple computer monitors


23

Sensors, Pixels and Image Sizes that had that setting. However, it no longer has any meaning except as an approximate average for all monitors. It may as well be 62 or 82. As you can see from the table below, images can be displayed at a variety of ppi—it all depends on the monitor, not the image. The table shows a range of ppi from 30 to 91 but more expensive displays may have even more pixels per inch. It’s best to forget the 72 ppi number and think of screen resolutions when sizing images for display on the screen. If their width and height in pixels is less than the screen’s resolution, they will be fully displayed. If they are larger, the viewer can only see part of the image at a time and will have to scroll around it—somewhat like reading a newspaper with a magnifying glass. For this reason, most images to be sent by e-mail or posted on a Web site are sized to the lowest possible common denominator—no larger than 600–800 pixels wide or 400–600 pixels high. The numbers in italics in this table are the pixels per inch for each combination of monitor screen width and resolution setting.

Monitor’s Horizontal Width Resolution

14”

15”

17”

19”

21”

640 x 480

46

43

38

34

30

800 x 600

57

53

47

42

38

1024 x 768

73

68

60

54

49

1280 x 800

91

85

75

67

61

When considering screen displays, one thing to think about is the aspect ratio.

Computer screens and those found on other digital devices generally have a standard resolution. It’s this resolution that determines how many pixels are used to display an image. Here are the names and sizes of resolutions offered on standard screens. Most monitors will support more than one resolution. Here are the names and sizes of resolutions offered on standard screens. Most monitors will support more than one resolution.

24

QVGA

320 x 200

SXGA+

1400 x 1050

EGA

640 x 350

UXGA

1600 x 1200

VGA

640 x 480

WSXGA+

1680 x 1050

SVGA

800 x 600

WUXGA

1920 x 1200

XGA

1024 x 768

QXGA

2048 x 1536

SXGA

1280 x 1024

QSXGA

2560 x 2048

WXGA

1366 x 768


Exploring Pixels on the Screen

Exploring Pixels

on the

Screen

Part 2 on the Excel worksheet “Pixels & Images Calculator” calculates the size of an image displayed on the screen. The numbers in the descriptions that follow refer to row numbers on the worksheet.

http://www.photocourse.com/itext/pixels/pixelcalc.xls Click to open an Excel worksheet and use Part 2. Displaying Images to follow the instructions in this section.

TIP To see what resolution your Windows system is set to: • On an XP system, right-click the desktop, click Properties, then click the Settings tab on the dialog box. • On a Vista PC rightclick the desktop, select Personalize, then click Display Settings.

1. Width of image (in pixels) is where you enter the image’s width in pixels. 2. Height of image (in pixels) is where you enter the image’s height in pixels. 3. Screen’s horizontal width (inches) is where you enter your screen’s width in inches (not its advertised diagonal measurement). 4. Screen’s horizontal resolution (pixels) is where you enter your screen’s horizontal resolution. For example, if resolution is set to 800 x 600, enter 800. If it’s set to 1024 x 768, enter 1024. 6. Screen’s PPI (pixels per inch) is calculated by dividing the screen’s horizontal resolution in pixels (line 4) by its actual width in inches (line 3). In the illustration above it divides 1024 by 16 for a ppi of 64. 7. Width of image on screen (in inches) is calculated by dividing the width of the image in pixels (line 1) by the screen’s ppi (line 5). In the illustration above it divides 3000 by 64 for an image width of 46.9 inches. 8. Height of the image on screen (in inches) is calculated by dividing the height of the image in pixels (line 2) by the screen’s ppi (line 5). 9. Will image fit on screen? is calculated by comparing the width of the image in pixels (line 1), with the screen’s horizontal resolution in pixels (line 4). If the image is equal to or smaller than the screen (in pixels), it will fit (YES), otherwise it won’t (NO). Exercises Open the worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the questions that follow. 1. If your image is 1600 x 1200 pixels, how wide will it be when displayed on your screen? Will it fit? 2. If your image is 1200 x 800 pixels, how large will it be when displayed on your screen? Will it fit? 3. Use Part of the worksheet to calculate the aspect ratios of the various screen displays illustrated on page 24.


25


Pixels

and

Print Sizes Printer resolutions are usually specified by the number of dots per inch (dpi) that they print while images and display screens are specified by pixels per inch ppi) For comparison purposes, monitors usually use somewhat less than 100 ppi to display text and images, inkjet printers range up to 4800 dpi or so, and commercial typesetting machines range between 1,000 and 2,400 dpi. Understanding printer resolutions is complicated by their advertised resolutions. The advertised dpi refers to the number of individual dots of ink the printer can print per inch. However, anywhere dots from 4 to 12 colors are needed to print a pixel of a specific color. For this reason, the ppi is always dramatically lower than the dpi. For most purposes, digital images print best at 200 or 300 ppi. Since image sizes are described in pixels and photographic prints in inches, you sometimes have to convert between these units. To do so, you divide the image’s dimension in pixels by the resolution of the device in dots per inch (dpi). For example, to convert the dimensions for a 1500 x 1200 image being printed at 300 ppi you divide as follows:

Width: 1500 pixels ÷ 300 dpi = 5” Height: 1200 pixels ÷ 300 dpi = 4” The result is a 5” x 4” print. However, if the output device prints 600 dpi, the print size falls to 2.5” x 2” as follows:

Width: 1500 pixels ÷ 600 dpi = 2.5” Height: 1200 pixels ÷ 600 dpi = 2”

This graphic shows the relative sizes of a 3000 x 2000 image printed or displayed on devices with different dots per inch. At 72 dpi it’s 41.7” x 27.8”, at 300 dpi it’s 10” by 7”, and at 1500 dpi, it’s only 2” x 1.3”—a little larger than a stamp.

26


Exploring Print Sizes

Exploring Print Sizes Part 3a on the Excel worksheet “Pixels & Images Calculator” calculates the http://www.photocourse.com/itext/pixels/pixelcalc.xls Click to open an Excel worksheet and use Part 3a. Printing ImagesPrint Sizes to follow the instructions in this section.

size of print you can expect from a given file size and the dpi you choose to print at. The numbers in the descriptions that follow refer to row numbers on the worksheet.

1. Width of image (in pixels) is where you enter the image’s width in pixels. 2. Height of image (in pixels) is where you enter the image’s height in pixels. 3. Printer’s resolution (in dpi) is where you enter the resolution your printer uses (this isn’t the same as the number of ink drops it sprays, and is usually set in a photo-editing program). 4. Width of print (in inches) is calculated by dividing the width of the image in pixels (line 1) by the dots-per-inch used to print it (line 3). 5. Height of print (in inches) is calculated by dividing the height of the image in pixels (line 2) by the dots-per-inch used to print it (line 3) Exercises Open the worksheet by clicking the Excel button in this sectioon and enter numbers in the green cells to explore the questions that follow. 1. If your image is 1600 x 1200 and you print it at 600 dpi, how big will the print be? 2. If your image is 800 x 600 and you print it at 300 dpi, how big will the print be? 3. If you print an image at 300 dpi, how wide will it have to be in pixels, to get a 6-inch wide print? 4. Using the original widths and heights listed below and the specified printer dpi’s, calculate the width and height of the prints you’d get. • Original 800 x 600, printed at 300 dpi is ____ x ____ • Original 800 x 600, printed at 600 dpi is ____ x ____ • Original 1600 x 1200, printed at 300 dpi is ____ x ____ • Original 1600 x 1200, printed at 600 dpi is ____ x ____ • Original 1800 x 1600, printed at 300 dpi is ____ x ____ • Original 1800 x 1600, printed at 600 dpi is ____ x ____


27


Pixels

per

Inch Normally you don’t have to change the number of pixel’s in an image to change the size of a printout. That task is handled by the software program you use to print the image. For example, if you place an image in a program such as QuarkXpress or PageMaker, it’s automatically printed at the size you specify in those programs. In Photoshop there are two ways you can change an image’s size—by changing the number of pixels in the image; or by changing the size of the area in which the available pixels are printed or displayed— the document size. These two procedures are separate but related.

In Photoshop, you can display the image size dialog box, turn off “Resample Image” and then specify any print size. The resolution of the image is calculated and displayed in pixels per inch. If it’s between 200–300, the results should be good on most inkjet printers.

TIP Imagine an image made up of dots printed on a rubber sheet. As you stretch the rubber to make the picture larger, the dots spread out lowering the pixels per inch.

TIPS • The document size

that you specify for an image determines its size if you copy or place it into a document created with another application.

• Some people

swear that when you enlarge an image by resampling it, you get better results if you enlarge it in 10% steps until it reaches the size you want.

28

• Pixel dimensions specifies the number of pixels an image contains. Initially determined by the number of pixels captured by the camera there are times you may want to change this size by deleting or adding pixels. For example, you may want to e-mail or post an image on a Web site. For this purpose it’s best if an image is no larger than the lowest common denominator screen, usually 640 x 480, or 800 x 600. Reducing an image’s size also makes the file size smaller so the image can be sent or displayed more quickly. The main reason you would increase the number of pixels in an image is to make large prints. Since most images print best when they are printed at 200-300 ppi you may get better results by enlarging the image rather than letting the pixels per inch fall below 200. To change the number of pixels in an image, you resample it to make it smaller by removing pixels, or larger by adding them. Reducing an image usually has less affect on its appearance than does enlarging one. This is because enlarging requires the program to add pixels—a process called interpolation. The computer analyzes adjoining pixels to determine the color of the new ones it inserts. Normally, you can double the size of an image without effects showing. However, trial and error is the only way to be sure because images vary so much. Look for the image becoming soft, as if it’s not sharply For more on textbooks in digital photography, visit http://www.photocourse.com

Pixels per Inch focused. If you are making other changes to the image, resampling it should be done after all other changes other than sharpening (page 73). This is because most adjustments work best where there are the maximum number of original pixels to work with.

TIP If you make any mistakes in the dialog box, hold down Alt (Option on Macs) to change the Cancel button to Reset and click it to start over.

• Document size specifies how large an image will be printed or displayed, especially in other applications. Normally you change the document size with resampling turned off. As a result, as the size increases, the pixels per inch decrease because the same number of pixels are spread over a wider area. If the resolution falls below 200 or so pixels per inch, you may want to consider resampling the image. There are problems printing with less than 200 pixels per inch and with resampling to increase the number of pixels so you’ll have to experiment to see which works best for a particular image. Just be sure your image is not too large to fit on the page. Many printers can’t print to the edge of the sheet so there is always a border. To print the full image, it must fit inside this border area. To change either the pixel dimensions or document size, select Image> Resize>Image Size in Photoshop Elements or Image> Image Size in Photoshop to display the Image Size dialog box having the following settings: • Pixel Dimensions shows the image’s Width and Height in pixels and next to the heading is the size of the image file. You can click the drop-down arrow to specify changes as a percentage. • Document size shows you the current Width and Height of the image in inches, centimeters, or any other unit of measurement you select with the drop-down arrow. Resolution displays the image’s pixels per inch at the current document size. This number changes as you change image width and height. If you make the image larger, the existing pixels are spread over a larger area so the pixels per inch decreases. The only way to change this relationship is to add more pixels to the image by resampling it. • Constrain Proportions check box determines if one of the photo’s dimensions will adjust automatically when you change the other. If you turn this off, the image’s proportions or aspect ratio changes and the image is stretched in one direction. Unless you are after a special effect, you normally leave this check box on. Chain link icons connecting the width and height settings indicate when this setting is on. • Resample Image check box determines if the number of pixels in the image will change when you change the size. When specifying a size for printing you usually turn this off. When you resample an image to add or subtract pixels, you can choose a process that trades off quality versus speed. Nearest Neighbor is fast but doesn’t give the best results, Bilinear is faster and gives better results, and Bicubic, the default, is slowest but best. One thing to keep in mind is that if you enlarge a print too much, it won’t be as sharp as you may desire. That’s because a certain minimal number of dots per inch, usually between 200 and 300 are needed to get a good print. Pixels begin to show when the print is enlarged to a point where the dots per inch (dpi) fall too low. If your printer prints the sharpest images at 300 dpi, you need to determine if the size of the image you plan on printing will fall below this level. To do so, you divide the chosen dpi by the width of the image in inches. For example, if you print an image that’s 1600 pixels wide so the print is 10” wide, there are only 160 dots per inch (1600 pixels ÷ 10 inches = 160 pixels per inch). However, if you print the same image so it’s 5 inches wide, the dots per inch climbs to over 300.


29


Exploring Pixels Per Inch Part 3b on the Excel worksheet “Pixels & Images Calculator” calculates the

http://www.photocourse.com/itext/pixels/pixelcalc.xls dpi of a print when you use a program that automatically resizes a file for Click to open an Excel worksheet and use Part 3b. Printing Images-DPI to follow the instructions in this section.

printing. Open the worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the questions that follow.

1. Width of image (in pixels) is where you enter the image’s width in pixels. 2. Height of image (in pixels) is where you enter the image’s height in pixels. 3. Desired width of print (in inches) is where you enter the width of the print you want in inches. 4. Height of print (in inches) is calculated so the print has the same aspect ratio as the image. The formula is in the form of a:b::c:d. 5. DPI (dots per inch) is calculated by dividing the width of the image in pixels (line 1) by the desired width in inches (line 3). Exercises Open the worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the questions that follow. 1. If you print a 4 x 6 image from an 1800 x 1600 file, how many dpi will there be along the long dimension? 2. If you print a 4 x 6 image from an 1800 x 1600 file, how many dpi will there be along the short dimension?

30


Pixels and Colors

Pixels

and

Colors Resolution isn’t the only factor governing the quality of your images. Equally important is the number of colors in the images. When you view a natural scene, or a well done photographic color print, you are able to differentiate millions of colors. Digital images can approximate this color realism, but whether they do so on your system depends on its capabilities and its settings. How many colors there are in an image, or how many a system can display is referred to as color depth, pixel-depth, or bit depth. Almost all newer systems include a video card and a monitor that can display what’s called 24bit true color. It’s sometimes called true color because these systems display 16 million colors, about the number the human eye can discern. How do bits and colors relate to one another? It’s simple arithmetic. To calculate how many different colors can be captured or displayed, simply raise the number 2 to the power of the number of bits used to record or display the image. For example, 8-bits gives you 256 colors because 28=256. Here’s a table to show you some other possibilities. Name

Bits Used

Black & white line art

1

GIF images

8

Formula 21

Number of Colors

28 224

256

JPEG images

24

Gray scale images

24

224

RAW images–12-bit

36

RAW Images–14 bit

42

236 242

2 16 million 16 million 68 billion 4 trillion

Black and white images require only 2-bits to indicate which pixels are white and which are black. Gray scale images need 8 bits to display 256 different shades of gray. Color images are displayed using 4 bits (16 colors), 8 bits (256 colors), 16 bits (65 thousand colors) called high color, and 24 bits (16 million colors) or true color. Some cameras and image formats use up to 42 bits per pixel (14 bits per color). These extra bits, trillions of them, are used to improve the color in the image as it is processed down to its 24-bit final form. In addition to affecting image quality, color depth also has an impact on file sizes. The more bits assigned to each pixel, the larger an image file becomes.


31


Exploring Pixels

and

Colors

Part 4 on the Excel worksheet “Pixels & Images Calculator” calculates the tohttp://www.photocourse.com/itext/pixels/pixelcalc.xls Click to open an Excel worksheet and use Part 4. Color Depth and File Sizes to follow the instructions in this section.

REVIEW: BITS AND BYTES • When reading

about digital systems, you frequently encounter the terms bit and byte.

• The bit is the smallest digital unit. It’s basically a single element in the computer that like a light bulb has only two possible states, on (indicating 1) or off (indicating 0). The term bit is a contraction of the more descriptive phrase binary digit.

• Bytes are groups of

8-bits linked together for processing. Since each of the eight bits has two states (on or off), the total amount of information that can be conveyed is 28 (2 raised to the 8th power), or 256 possible combinations.

tal number of pixels in an image when you enter the image’s width and height in pixels. Open the worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the questions that follow.

1. Bits per color is where you enter the number of bits your image uses for each color—red, green, and blue. 2. Bits per pixel is calculated by a formula that multiplies bits per color (line 1) by 3 since three colors are used for each pixel. 3. Number of possible colors is calculated by raising the number 2 to number of bits per pixel (line 2). 4. Width of image (in pixels) is where you enter the image’s width in pixels. 5. Height of image (in pixels) is where you enter the image’s height in pixels. 6. Total number of pixels in image is calculated by multiplying the image’s width by its height. 7. File size (uncompressed) is calculated by multiplying the total number of pixels in the image by the number of bits used to store each pixel. File sizes are shown in bits, bytes, kilobytes, and megabytes. 8. Table of color depths shows color depths used by various image types. Exercises Open the worksheet by clicking the Excel button in this section and enter numbers in the green cells to explore the questions that follow. 1. If an image assigns the following number of bits to each pixel, how many colors can be displayed? • 2 bits = __________ colors

• 8 bits = __________ colors

• 16 bits = __________ colors

• 24 bits = __________ colors

• 32 bits = __________ colors

• 36 bits = __________ colors

2. If an image is 3000 x 2000 pixels and 32 bits of color, how large is the file in megabytes? _________ 32


Scanning and Image Sizes

Scanning

and

Image Sizes Color scanners work by creating separate red, green, and blue versions of

http://www.photocourse.com/itext/pixels/pixelcalc.xls the image, and then merging them together to create the final digital imClick to open an Excel worksheet on scanning and follow the instructions in this section.

age. Some scan all of the colors in one pass while others take three passes, a slower but higher quality method. Which method is used depends on the scanner’s image sensor. Most scanners use linear CCDs arranged in a row. Those that require three passes use a single row of photosites and pass different filters (red, green, or blue) in front of the sensor for each pass or use three different light sources. Others have 3 rows of photosites, each row with its own filter so they can capture all three colors on a single pass. As the image is scanned, a light source travels down the photo (some print and document scanners instead move the document past the light source). The light source reflects off a print or passes through a transparency and is focused onto the image sensor by a mirror and lens system. Because of this mirror and lens system, the sensor does not have to be as wide as the area being scanned. The horizontal optical resolution of the scanner is determined by the number of photosites on its sensor. However, the vertical resolution is determined by the distance the paper or light source advances between scans. For example, a scanner with a resolution of 600 x 1200 has 600 photosites on its sensor and moves 1/1200 of an inch between each scan. Scanning and file sizes When scanning, your goal is to get a digital image file that contains all of the detail you need without the file being too large to work with. If you scan at too low a resolution, you’ll lose detail. If you scan too high, you’re file will be too large. When you scan an original image—either a slide or a print—the file size depends on a number of factors including the area being scanned, the resolution of the scanner, and the color depth, or number of bits assigned to each pixel. Let’s take a look at this step-by-step as you’d calculate file sizes using the downloadable scanning calculator—Part 1.

1. Enter the width in inches of the art to be scanned. 2. Enter the depth or height of the art to be scanned. For more on digital photography, visit http://www.shortcourses.com

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Sensors, Pixels and Image Sizes 3. Enter the scanner’s optical resolution or the resolution to intend to scan at. 4. The scanned pixels horizontally are calculated by multiplying the scanner’s resolution (line 3) times the horizontal size of the original (line 1). 5. The scanned pixels vertically are calculated by multiplying the scanner’s resolution (line 3) times the vertical size of the original (line 2). 6. The total scanned pixels is calculate by multiplying the horizontal scanned pixels (line 4) times the vertical scanned pixels (line 5). 7. The color depth is where you enter the number of bits assigned to each pixel. This would usually be 1 if the image is black & while, 8 if it’s grayscale (like a black and white photography), or 24 if it’s color. 8. The file size in bits is calculated by multiplying the number of pixels in the image (line 6) time the color depth (line 7). 9. The file size in bytes is calculated by dividing the file size in bits (line 8) by 8. 10. The file size in kilobytes is calculated by dividing the file size in bytes (line 9) by 1,000. 11. The file size in megabytes is calculated by dividing the file size in kilobytes (line 10) by 1,000. Scanning an image for printing at a specified size There are times when you know what size you want a print to be and need to calculate backwards to what size the image file should be. For most purposes, you can expect to get photorealistic quality with a print having about 300 dpi. This means, a 4 x 6 print needs an image file of 1200 x 1800, and an 8 x 10 print needs one 2400 x 3000. You can stretch or shrink images you’ve imported into programs such as PageMaker and QuarkXPress. Because the number of pixels in the image don’t change, the dots per inch have to. For example, let’s say you placed an 800x600 image on a page and it was 2-inches wide. If you print it out at that size, it will print at 400 dpi (800 divided by 2”). If you now stretch the image to be 4 inches wide, the dpi drops to 200 (800 divided by 4”). If you want the image 4” wide AND 400 dpi, best scan it so it’s 1600 pixels wide before you import it. However, if it’s to be printed on a printing press, the rules change and become more complex. You should scan the image so its sots per inch are twice the lines per inch (lpi) at which it will be printed.

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Scanning and Image Sizes

1. Enter the width in inches of the art to be scanned. 2. Enter the depth or height of the art to be scanned. 3. Enter one of the the printer’s output resolution in dots per inch (dpi). 4. The horizontal size of the original is calculated by multiplying the desired horizontal size of the output (line 1) by the desired output resolution (line 3). 5. The vertical size of the original is calculated by multiplying the desired vertical size of the output (line 2) by the desired output resolution (line 3). 6. Enter the color depth of the image. This would usually be 1 if the image is black & while, 8 if it’s grayscale (like a black and white photography), or 24 if it’s color. 7. The file size in bits is calculated by multiplying the horizontal by the vertical size of the original (lines 4 and 5) to calculate the total number of pixels in the image and then multiplying those by the color depth (line 6). 8. The file size in bytes is calculated by dividing the file size in bits (line 7) by 8. 9. The file size in kilobytes is calculated by dividing the file size in bytes (line 8) by 1,024. 10. The file size in megabytes is calculated by dividing the file size in kilobytes (line 9) by 1,000. Scanning an image for screen display Scanning an image for the screen is the same as scanning one for printing except the output is usually specified in pixels, not inches. Although the actual number of pixels per inch on a monitor vary depending on its size and resolution, images are generally scanned at 72 ppi for screen display (although they are sometimes scanned up to 96 ppi). Making them any larger doesn’t add any information to the image and just makes the files larger.


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1. Enter the width in inches of the image to be scanned. 2. Enter the depth or height of the image be scanned. 3. Enter the screen’s resolution in dots per inch (dpi). This is normally 72 dpi on average. 4. Enter the desired width of the image in pixels. 5. The vertical size of the image is calculated by dividing it’s width on line 4 by the ratio of the original’s width to height calculate by dividing line 1 by line 2. 6. Enter the color depth of the image. This would usually be 1 if the image is black & while, 8 if it’s grayscale (like a black and white photography), or 24 if it’s color. 7. The file size in bits is calculated by multiplying the horizontal by the vertical size of the original (lines 4 and 5) to calculate the total number of pixels in the image and then multiplying those by the color depth (line 6). 8. The file size in bytes is calculated by dividing the file size in bits (line 7) by 8. 9. The file size in kilobytes is calculated by dividing the file size in bytes (line 8) by 1,024. 10. The file size in megabytes is calculated by dividing the file size in kilobytes (line 9) by 1,000.

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The Complete Excel Worksheet

The Complete Excel Worksheet Pixels & Images Calculator

from "The Textbook of Digital Photography" The #1 Educational Site in Digital Photography http://www.photocourse.com

1. Image Sizes and Aspect Ratios 1 2 3 4

Width of image (in pixels) Height of Image (in pixels) Total number of pixels in image Aspect ratio

2. Displaying Images 1 2 3 4 5 6 7 8

Width of image (in pixels) Height of image (in pixels) Screen's horizontal width (in inches) Screen's horizontal resolution (in pixels Screen's ppi Width of image on screen (in inches) Height of image on screen (in inches) Will image fit on screen?

3a. Printing Images-Print Sizes 1 2 3 4 5

Width of image (in pixels) Height of image (in pixels) Printer's resolution (in dpi) Width of print (in inches) Height of print (in inches)

3b. Printing Images-Pixels Per Inch 1 2 3 4 5

Width of digital image (in pixels) Height of image (in pixels) Desired width of print (in inches) Height of print (in inches) DPI (dots-per-inch)

4. Color Depth and File Sizes 1 2 3 4 5 6 7

Bits per color Bits per pixel Number of possible colors Width of image (in pixels) Height of image (in pixels) Total number of pixels in image File size (uncompressed)

8 Table of color depths Name Bits per pixel Black and white images 1 Gray scale/GIF images 8 High color 16 True color/JPEG Images 24 RAW and TIFF 48

3,000 2,000 6,000,000 1.50

pixels pixels pixels to 1

3,000 2,000 16 1,024 64.0 46.9 31 NO

pixels pixels inches pixels ppi inches inches

1,600 1,200 200 8.00 6.00

pixels pixels dpi inches inches

1,600 1,200 6 4.5 267

pixels pixels inches inches dpi

8 24 16,777,216 1,600 1,200 1,920,000 46,080,000 5,760,000 5,625 5.5

Enter data only in the light green boxes (the ones with background colors matching this box's.)

pixels pixels pixels bits bytes kilobytes megabytes

(8 for each color-R, G, and B) (16 for each color-R, G, and B)


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