There comes a point at which adding more blades to a razor does not make shaving any better. Now that photographic film is pretty much ancient history, we have reached a point at which adding megapixels to a digital camera does not really make taking pictures any better. So if you're buying a new camera, megapixel count should probably be the least important factor in making your decision.
Shoppers at the Best Buy in Dearborn still discuss megapixel counts with the sales personnel as they mull over which camera to buy from the vast array of choices, but the fact is that almost any camera they buy there will have at least a good enough megapixel count. Anything over 6 megapixels will do for most purposes.
Consider for example that 1080I, "high definition" for digital television, is just a little tad more than 2 megapixels (multiply 1,920 by 1,080): this means that to show an image from most new cameras today on a TV screen requires scaling down the image. Today's computer screens have similar pixel dimensions, so to view an image in excess of 2.1 megapixels at 100% (no scaling) requires scrollbars. As the image gets bigger, the scrollbars get very tiny. The photo above is an 8.2 megapixel image that has been cropped, and it still needs to be scaled down by at least half to show all of it on a typical computer monitor.
Also consider file sizes, since many images taken with digital cameras have to go through a computer, especially if they are to be shared on the Web. For example, images on Examiner.com have to be "under 512kb [half a megapixel] and dimensions of 250x200 or larger," and there is a preference for images that are 400 by 324, which works out to about a tenth of a megapixel. This is enough to show some detail but not so much that people have to wait a long time for the page to load. Trying to upload even a 6.3 megapixel picture to Facebook is an exercise in patience.
Even with compression schemes (in which the camera or computer uses a shorthand to write fewer pixels), more megapixels usually means more megabytes, unless you're shooting fields of perfectly solid colors. As a rule of thumb, the number of megabytes is just short of half the number of megapixels, e.g., an image from a Canon Digital Rebel XT (8.2 megapixels) might be 3.5 megabytes, while an image from a Nikon D800 (36 megapixels) might be 17 megabytes.
Theoretically, more megapixels means sharper images. But professional photographers know that there are many other factors that affect image sharpness. Most obviously, all the megapixels in the world are worthless if the camera isn't focussed at all. Ease of focusing manually, and automatic focus accuracy are therefore much more important factors in choosing a camera than the metric of megapixels. The controls in general need to make sense to you; it would not be a good idea to switch from one brand of camera you're already familiar with to one you've never used before just to chase a few more megapixels.
It is nice to be able to shoot a photo focused on one particular thing and have it be sharp enough that you can zoom in on anything else in the image and still have it be sharp. But professional photographers with discipline from the old days of film have a tendency to shoot "full frame" as much as possible―that is, they compose the image in such a way that they don't have to crop anything later. It was a mark of pride back then to print not just the image but also the film sprockets, to prove that you hadn't cropped at all.
Just because your camera can shoot 20 megapixels doesn't mean you have to always shoot 20 megapixels. Canon cameras, for example, offer at least six quality settings: large fine, large choppy, medium fine, medium choppy, small fine and small choppy (these are not official terms, since the user manuals always use a combination of the letter L, M or S and an icon of a smooth quarter circle and a blocky quarter circle). The Rebel T3i goes so far as to offer four different small quality settings. But it would be nonsensical to buy a camera only because of its high megapixel count and then always use its small or medium setting.
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