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When buying a digital camera, many people focus on just one factor: megapixels, the more the better. But that's not the whole story. Lens quality, image-processing capability and even the size of the pixels can all have a greater effect on how your pictures turn out. "The number of megapixels," says Bob Sobol, an image scientist at Hewlett-Packard, "is relatively unimportant."

What the quantity of megapixels (each one equals 1 million pixels) does determine is how big you should make your prints. For most consumers, a bottom-of-the-line, 2-megapixel model is just fine for producing great 4-in. by 6-in. prints. Opt for 3 to 4 megapixels if you want to print 8-by-10s. You need the newer 5- or 8-megapixel models only if you want to create poster-size prints or plan to crop and zoom in your pictures on your computer before you print them.

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If you were to crack open your digital camera, one thing you would find is the image sensor, a tiny silicon chip about a half-inch wide embedded with millions of pixels tightly packed together. When struck by light, each pixel generates an electric current that is converted into the digital data that make up your picture. But not all pixels are created equal, and some cameras use larger ones than others. For example, the pixels on the HP Photosmart R707 are just 2.8 microns wide, whereas those on the Nikon D70 are 7.8 microns wide. (A micron is tiny—1/24,500 of an inch.) The advantage of a larger pixel is that it is able to pick up more information about the image it is sensing.

Another factor affecting quality is the brains of your camera: it needs to have enough processing power to turn raw data into a beautiful picture. The quality of the lens is also important to ensure that your picture is focused and to pull in enough light to get a good exposure. "If you have a bad lens, the number of megapixels doesn't mean anything," says Atsushi Tashiro, a product-development manager for Fujifilm.

How do you find the camera that's right for you? Read independent reviews on sites like cnet.com, dpreview.com and dcmag.co.uk. Then the next time someone brags about his new multi-megapixel camera, you can ask him how big his pixels are or what kind of lens he's got. Chances are, he'll have no idea.

• Anita Hamilton
• The Myth of Megapixels