If you haven't already guessed, the term image size describes the physical dimensions of an image. Resolution is the number of pixels per linear inch in the final printed image. I say linear because you measure pixels in a straight line. If the resolution of an image is 72 ppi — that is, pixels per inch — you get 5,184 pixels per square inch (72 pixels wide x 72 pixels tall = 5,184).
Assuming the number of pixels in an image is fixed, increasing the size of an image decreases its resolution and vice versa. An image that looks good when printed on a postage stamp, therefore, probably looks jagged when printed as an 11 x 17-inch poster.
Figure 3-1 shows a single image printed at three different sizes and resolutions. The smallest image is printed at twice the resolution of the medium-sized image; the medium-sized image is printed at twice the resolution of the largest image.
One inch in the smallest image includes twice as many pixels vertically and twice as many pixels horizontally as an inch in the medium-sized image, for a total of four times as many pixels per square inch. Therefore, the smallest image covers one-fourth the area of the medium-sized image.
The same relationships exist between the medium-sized image and the largest image. An inch in the medium-sized image comprises four times as many pixels as an inch in the largest image. Consequently, the medium-sized image consumes one-fourth the area of the largest image.
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