1) Megapixels Lie.
The more megapixels a camera can capture, the better the pictures, right? Wrong.
The logic seems reasonable: Since a digital picture is made up of dots (pixels), the more the camera can record, the finer the detail should be. Of course, you'll get better pictures with an 8-megapixel camera than with a 2-megapixel camera. But we've tested many cameras whose resolution (that is, ability to reproduce detail) is lower than you'd expect from their pixel count -- 10MP cameras that resolve only at the same level as 6MP cameras, for instance. And the reverse: We've seen 5MP cameras with the resolution of a typical 6MP camera.
The fact is, photographs are more than dots and details. They are the sum of their color, contrast, exposure, grain, and dynamic range. These qualities come not just from pixel count but from the design of the individual pixels and the camera's image processing -- in particular, how it deals with color reproduction and noise suppression. We've tested cameras that have spectacular resolution but atrocious noise levels that made for ugly, mottled pictures. The reason? The extreme sharpening used by such cameras to accentuate resolution also sharpens the noise.
Pop Photo Tip: Put less stock in the number of megapixels than in our Certified Test Results for image quality. What goes into that? Keep reading.
2) Image Quality: Testing is Believing.
Our most critical tests of DSLRs are resolution (capture of fine detail), color accuracy (the ability to closely reproduce the colors in a scene), and noise (the level of unwanted signal in the picture, which shows up as digital grain and, in bad cases, mottling and color blotches). A camera that tests with Excellent resolution, Excellent color accuracy, and no greater than Low noise will get an Excellent overall image quality rating.
An important point: ISO settings influence camera performance. When you turn up the ISO (sensitivity) on a DSLR, it boosts the signal from the pixel array. This also tends to boost the unwanted signal -- noise. Camera makers use various strategies to squelch noise at higher ISOs, commonly using a blurring filter to smooth over the detail and thus tone down the graininess. But this also lowers resolution. That's why we give image quality ratings for a range of ISOs.
We also evaluate dynamic range (the ability to record a wide highlight/shadow range) and image contrast. And we take a lot of pictures of all sorts to confirm our data with real-world shooting.
Pop Photo Tip: While our overall image quality ratings give you performance at a glance, consider the finer test details, too. If you do a lot of available-light shooting, you may be more concerned with good noise suppression at high ISOs than with resolution. If you do mostly portraits, you'll value high color accuracy, good dynamic range, and controllable contrast.