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'd suggest you forget about 72 dpi. We hear a lot of things that just don't stand up well to examination.
There is no way to use notions of 72 dpi to produce a useful result (I bet you already knew that), because video simply doesn't work that way. There is no concept of dpi in the video system. Video systems show pixels, one for one. If you scan at 72 dpi, what you get is 72 pixels per inch of original photo dimension. That may or may not be the size of image you want. Often it is not (even if it were, 75 dpi would be better, see integer divisors).
The only one possible virtue of the concept of scanning at 72 to 96 dpi is that it will create an image size in pixels that is usually a rough approximation of original size in inches on many common monitors. However it is simply not accurate, the same image will still appear at different sizes on different screens (as stated before).
Even if accurate original size were possible (on paper yes, but it is not possible on the screen), would you even want it? Scan something small at 72 dpi, a postage stamp, or a 35 mm film frame. All you get is a small thumbnail image. Then try 600 or 1200 dpi. See the difference? There is a HUGE difference, and there are many choices, and the previous page showed how to accurately predict precisely what will happen, so you can get any result that you want.
For the screen, scanning at 72 ppi is simply one of many choices, and it produces one specific image size (72 pixels per inch of original). On most commonly used screen sizes, scanning at 70 to 100 ppi creates images usually seen as about original size in inches (very roughly). Most desktop video screens today are closer to 96 dpi, but they vary. However, exact original size on the screen is not a valid concept, because screen sizes simppy vary, so any image varies in size seen on different screens.
There are of course many other size choices than 72 ppi. If 72 ppi is the image size you want on the screen, that's fine, but try scanning at 75 ppi, the scanner can do slightly better (Chapter 9). But if this is not the size you want, then forget about 72 ppi, it has no significance at all.
The loss of the false 72 dpi myth can be pretty earth-shaking for some, so if it's a problem, here is more elaboration.
Video monitors are relatively low resolution devices. A 17 inch monitor screen might measure 12.5 inches horizontally. If it is set to 1024x768 pixel screen size, then the image is obviously 1024 pixels / 12.5 inches = 82 dpi apparent resolution in that case (if we had an image 82 pixels wide, it would appear as one inch on that one screen). A 15 inch monitor at 800x600 pixels might be 75 dpi. A 14 inch monitor at 640x480 pixels might be about 65 dpi. Other sizes and settings compute other numbers, but most combinations normally used are vaguely in the rough range of 72 to 96 dpi.
This computation is the origin of those numbers, and their only significance. Screens are NOT 72 dpi in any way except this one way, which is not a factor in scanning for the screen (an effect, not a cause). We may compute that apparent dpi number, but the video system had no concept of it. The only important factor is the size of the screen, like 1024x768 pixels, and how your image size fits in those 1024x768 pixels.
The concept of those calculated dpi numbers has little significance to scanning. Neither the video driver nor the video board has any concept of screen size in inches, and therefore dpi can have no meaning for the images either. We can compute that apparent dpi number, but again, the video system had no concept of it. The video system does not use those numbers, video simply shows those 1024x768 pixels directly. You can scan at that computed dpi number to show actual original size on that one computed screen, but that's generally pointless, because this image size (in inches) would not be the same on a second monitor, screens vary in size.
This section is about video screens. Screens only show pixels, directly, and screens vary in size. This is an extremely important and fundamental concept. You won't make much progress without it. And the best part is that the right answer is extremely simple, the whole story is here. See how to use it on the next page.