How to scan photos for the Web: Debunking the 72-DPI myth
by Chris Nicholson

The first question many digital newcomers ask when scanning an image for computer use is: What DPI should the image be?

The answer these people usually get is: 72 DPI.

However, the correct answer is: It really doesn't matter.

This can cause a lot of controversy, because many people — even ones who otherwise really know what they're talking about — will insist that 72 DPI is the correct photo resolution for a computer screen. But it's not.

I promise you: DPI is an overrated concept when it comes to images that are solely for use on a computer. If you plan to print your photo, or to send it for separation for use in a publication, then resolution is a critical detail. But if you intend to use an image only as decoration on your computer's desktop, to post it on the Internet or to e-mail it to a friend, then you don't even need to know what DPI is.

Look at the two photos below and try to guess the DPI value of each.

Bull calf, near Houston, Texas. Nikon F5, Nikkor 80-200mm f/2.8. More Animal Pictures.

Same bull. Same camera.

The answer: The photo on the left is 100 DPI. The photo on the right is 1 DPI. That's not a typo; it is one DPI. Download it and see for yourself. The photos appear the same because: On a computer screen, DPI doesn't matter.

Explanation

DPI is the abbreviation for "dots per inch." But a monitor doesn't know what an inch is, so how many dots are in one is irrelevant to the computer. All the monitor wants to know is the total number of dots — or pixels — that are in the image. A photo could be 5 inches wide at 100 DPI, or could be 10 inches wide at 50 DPI; either way the computer only cares that the photo is 500 pixels wide, so it can use 500 pixels to display it.

The size of an image on the monitor will then vary with how the monitor is configured. For example: Suppose you have a 17-inch monitor set to a resolution of 800X600. If you open a 400x300-pixel image, that image will take up one-quarter of the display. But if you set that same 17-inch monitor to a resolution of 1280x1024, then that same 400X300 image will fill only about one-tenth of the display. But the DPI value is never considered in these calculations, because, again: The only measurement a monitor cares about is pixel dimensions.

So the real question when scanning for the Web is: How many pixels wide and tall should I make the photo? The answer: It depends on your taste.

Just realize that the image will appear at different sizes on different computers, based on the size of the monitor and its resolution. So make it 500x500 pixels, or 50x50, or whatever you want. For my sites, I make horizontal images about 360 pixels wide, vertical images 240 pixels tall. I find that those sizes appear not too large on 800x600 monitor displays, but not too small for 1280x1024.

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