David Pogue's Digital Photography: The Missing Manual

Megapixels

The first number in the description of a camera is usually how many megapixels it has. And that's probably the least important detail of the camera.

A pixel (short for picture element) is one tiny colored dot, one of the thousands or millions that compose a single digital photograph.

You need at least a million pixels—that is, 1 megapixel—to make a 4 x 6-inch print. Much fewer than that, and the print might look a little coarse; you might be able to see the individual dots. That's why we have the shorthand: Instead of saying that your camera has 8,100,000 pixels, you'd say that it's an 8.1-megapixel camera.

What you're describing is its resolution. For instance, an 8-megapixel camera has a higher resolution than a 4-megapixel camera. (It also costs more.)

So how many pixels do you need?

• Pictures on the screen. Many photos are destined to be shown solely on a computer screen: to be sent by email, posted on a Web page (like eBay), turned into a screen saver, or used as a desktop picture.

  If this is what you have in mind, congratulations. You're about to save a lot of money, because you don't need a lot of megapixels at all. Even 2 megapixels (as you might find on a cameraphone) is way, way too big for use in email or on an auction site. In fact, it's probably about 1600 x 1200 pixels—even too big to fit on a typical 12-inch laptop screen without zooming out or scrolling.

• Pictures on paper. If you intend to print your photos, however, it's a different story. A printer must cram ...