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1.6 IMAGE SIZES | ||
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The quality of any digital image, whether printed or displayed on a screen, depends in part on the number of pixels it contains. More and smaller pixels add detail and sharpen edges.
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| The number of pixels in an image determines its size and can have an affect on its resolution. |
This table lists some standards of comparison. The numbers from various sources differ. One great thing about the Web is that you can talk back to an author and correct him. Click here to send a message setting me straight.
| Element | Pixel Count | Total Pixels |
| Color TV (NTSC) | 320 x 525 | 168,000 |
| Human eye | 11,000 x 11,000 | 120 million |
| 35-mm slide | The "Economist" magazine says it has 20 million or more. CMOS Imaging News says 5 to 10 million depending on the film. Another source says about 80 million pixels. Robert Caspe at SoundVision states that color negative film has 1000 pixels per inch while color positive film has 2000 pixels per inch. | |
| 1982 Kodak Disc camera film | 3 million pixels—each about 0.0003 inch in diameter |
| The Collision of Two Worlds
The term "resolution" was introduced in the computer world as a way to describe screen displays. In the early days, a screen would have a CGA or VGA resolution. Later, other names were introduced to describe even larger screens. The terms were used to describe the number of pixels on the screen. For example, the VGA screen had 640 pixels across the screen and 480 down (640 x 480). No one was concerned about the use of the term at the time it was introduced. It's only when photography became digital that another group of people entered the scene with a totally different use of the term. To photographers, or anyone in optics, resolution describes the ability of a device to resolve lines such as those found on a test chart.
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When you read ads for imaging equipment, you sometimes have to read the fine print when it comes to "resolution." There are two kinds; optical and interpolated. The optical resolution of a camera or scanner is an absolute number because an image sensor's photosites are physical devices that can be counted. To improve resolution in certain limited respects, the number of pixels in an image can be increased using software. This process, called interpolated resolution, evaluates those pixels surrounding each new pixel to determine what its colors should be. For example, if all of the pixels around a newly inserted pixel are red, the new pixel will be made red. This same thing can be done in a photo editing program such as Photoshop by resizing the image. 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. In fact the one thing it doesn't have any effect on is resolution in it's true sense—the ability to resolve closely spaced lines.
Size isn't everything!The higher a camera's pixel count, the larger the image files that it creates. For this reason, some cameras allow you to specify more than one size when you take a picture. Although you are likely to get better results with a larger image, it isn't always needed—especially when the image is going to be displayed on the Web or printed very small. In these cases smaller images will suffice and because they have smaller file sizes, you'll be able to squeeze more into the camera's memory. |
There is only one way to increase optical resolution, add more photosites to the sensor. Doing so isn't easy and creates other problems. For example:
- It requires that the sensor chip be larger and/or each photosite smaller. Larger chips with more photosites increase difficulties (and costs) of manufacturing. Smaller photosites must be more sensitive to capture the same amount of light.
- More photosites create larger image files, creating storage problems.
The resolution of a display monitor is almost always given as a pair of numbers that indicate the screen's width and height in pixels. For example, a monitor may be specified as being 640 x 480, 800 x 600, 1024 x 768, and so on.
- 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.
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This is a 640 x 480 display. That means there are 640 pixels on each row and there are 480 rows. |
Images displayed on the monitor are very low-resolution. As you can see from the table below, the actual number of pixels per inch depends on both the resolution and the size of the monitor. Generally, images that are to be displayed on the screen are converted to 72 pixels per inch (ppi), a resolution held over from an early era in Apple's history. (The red numbers in the table are the pixels per inch for each combination of screen size and resolution.) As you can see from the table, this isn't an exact number for any resolution on any screen, but it tends to be a good compromise.
To understand the table, consider and example. You have two screens, each set to 800 x 600 resolution. One screen is 14" and one is 21". An 800 x 600 pixel image will fill either screen. However, on the larger monitor the 800 pixels on each row of the image are spread across a wider screen. It's for this reason that the pixels per inch decrease.
| Resolution | Monitor Size |
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| 14 | 15 | 17 | 19 | 21 | |
| 640 x 480 | 60 | 57 | 51 | 44 | 41 |
| 800 x 600 | 74 | 71 | 64 | 56 | 51 |
| 1024 x 768 | 95 | 91 | 82 | 71 | 65 |
Printer resolutions are usually specified by the number of dots per inch (dpi) that they print. (Generally pixels per inch refer to the image and display screen and dots per inch refer to the printer and printed image. Sometimes I think terminology shifts like this are done just to confuse us. In this book we use them interchangeably) For comparison purposes, monitors use an average of 72 ppi to display text and images, ink-jet printers range up to 1700 dpi or so, and commercial typesetting machines range between 1,000 and 2,400 dpi.
