Why 72 dpi?
We often hear that we should scan at 72 dpi for the video screen, like it’s some kind of magic number. It’s not. I know a few of the best sources tell us this, and I can’t defend them, but it’s very much less than the whole story. I believe an early Apple model described a 72 dpi monitor at one time, back before screen dimensions were adjustable, and we’re stuck with it now, even without other evidence. There is nothing we can do with that number. For one thing, monitors vary today. A 15 inch monitor screen might measure 10.6 inches horizontally (the actual bright image area). If it is currently set to 800×600 pixels screen size, then the image is obviously 800 dots / 10.6 inches = 75 dpi apparent resolution in that case. Reset the same monitor to different screen sizes, and we get different numbers. Monitors vary, and the horizontal size control even changes it too.
