Digital images are made up of numbers. The fundamental particle of a digital image is the pixel, and the number of pixels you capture determines the image’s size and aspect ratio. It’s tempting to use the term resolution, but doing so often confuses matters more than it clarifies them. Why?
Pixels and Resolution
Strictly speaking, a digital image in its pure Platonic form doesn’t have resolution—it simply has pixel dimensions. It only attains the attribute of resolution when we realize it in some physical form—displaying it on a monitor or making a print. But resolution isn’t a fixed attribute.
If we take as an example a typical 6-megapixel image, it has the invariant property of pixel dimensions: specifically, 3,072 pixels on the long side of the image and 2,048 pixels on the short one. But we can display and print those pixels at many different sizes. Normally, we want to keep the pixels small enough that they don’t become visually obvious—so the pixel dimensions essentially dictate how large a print we can make from the image. As we make larger and larger prints, the pixels become more and more visually obvious until we reach a size at which it just isn’t rewarding to print.
Just as it’s possible to make a 40-by-60-inch print from a 35mm color negative, it’s possible to make a 40-by-60-inch print from a 6-megapixel image, but neither of them is likely to look very good. With the 35mm film, you end up with grain the size of golf balls, and with the digital capture, each pixel winds up being just under 1/50th of an inch square—big enough to be obvious.
Different printing processes have different resolution requirements, but in general, you need no fewer than 180 pixels per inch, and rarely more than 480 pixels per inch, to make a decent print. So the effective size range of our 6-megapixel capture is roughly from 11 by 17 inches downward, and 11 by 17 is really pushing the limits. The basic lesson is that you can print the same collection of pixels at many different sizes, and as you do so, the resolution—the number of pixels per inch—changes, but the number of pixels does not. At 180 pixels per inch, our 3072-by-2048-pixel image will yield a 17.07-by-11.38-inch print. At 300 pixels per inch, the same image will make a 10.24-by-6.83-inch print. So resolution is a fungible quality: you can spread the same pixels over a smaller or larger area.
To find out how big an image you can produce at a specific resolution, divide the pixel dimensions by the resolution. Using pixels per inch (ppi) as the resolution unit and inches as the size unit, if you divide 3,072 (the long pixel dimension) by 300, you obtain the answer 10.24 inches for the long dimension. If you divide 2,048 (the short pixel dimension) by the same quantity, you get 6.826 inches for the short dimension. At 240 ppi, you get 12.8 by 8.53 inches. Conversely, to determine the resolution you have available to print at a given size, divide the pixel dimensions by the size, in inches. The result is the resolution in pixels per inch. For example, if you want to make a 10-by-15-inch print from your 6-megapixel, 3,072-by-2,048-pixel image, divide the long pixel dimension by the long dimension in inches, or divide the short pixel dimension by the short dimension in inches. In either case, you’ll get the same answer: 204.8 pixels per inch.
Figure 2-1 shows the same image printed at 50 pixels per inch, 150 pixels per inch, and 300 pixels per inch.
Figure 2-1 Image size and resolution.
Each pixel is defined by a set of numbers, and these numbers also impose limitations on what you can do with the image, albeit more subtle limitations than those dictated by the pixel dimensions.