People want things to be simple. People want one number to tell them whether one camera is better than another. Camera manufacturers are well aware of this and proudly play the "megapixel war game". So, I will say it again a different way: The number of megapixels has very little to do with image quality.
I learned about megapixels not being everything when I was offered a 5 megapixel HP point and shoot camera. I was considering buying it to replace (upgrade) a 2 megapixel Canon point and shoot camera. I shot some photos with the HP and was appalled at the amount of digital noise. The Canon had less than half the megapixels, but they were clearly much better pixels! (And no, I didn't buy the camera).
A good use of the pixel count is to predict what size print can be produced from the images taken by a given camra.
Here are some guidelines offered by Scott Kelby in "The Digital Photography Book", volume 1, page 182.
3 megapixels -> 5x7 print 4 megapixels -> 8x10 print 5 megapixels -> 11x14 print 6 megapixels -> 13x19 print 8 megapixels -> 16x20 print 10-12 megapixels -> 24x36 printThe moral of the story is that you can probably save yourself a lot of money if you only ever intend to make 8x10 prints or images for the web (which is the case for a lot of people).
I found these numbers suprising, but I have found Scott to be right on about virtually everything he says. I had the theory that these values might be based on some kind of target pixel density on the final print (like 300 dpi), so I performed the following calculations (which did not pan out):
3 megapixels -> 5x7 print -> 35 sq inches -> .086 psi -> 292 dpi 4 megapixels -> 8x10 print -> 80 sq inches -> .050 psi -> 224 dpi 5 megapixels -> 11x14 print -> 154 sq inches -> .032 psi -> 180 dpi 6 megapixels -> 13x19 print -> 247 sq inches -> .024 psi -> 156 dpi 8 megapixels -> 16x20 print -> 320 sq inches -> .025 psi -> 158 dpi 10-12 megapixels -> 24x36 print -> 865 sq inches -> 0.13 psi -> 113 dpiApparently Scott's guidelines are based on his own experience with what is required to make quality prints rather than a 300 dpi rule. And there is a good justification for requiring lower pixel densities for larger prints as they are generally viewed from a greater distance.
Indeed 300 dpi is a commonly quoted "industry standard" for the pixel density required of a print used for publication. It is claimed that 200-250 dpi is adquate for prints being made for personal use. In fact I see the claim being made that 150 dpi gives great results with inkjet printers. One guide calls 300 dpi "photo quality", 150 dpi "poster quality", and 40 dpi "billboard quality". The real numbers will vary with the viewing distance, the print process, the subject, and the expecations of the viewer.
Until I have more personal experience I will accept Scott's guidlines. I also respect his advice about not spending money for resolution that you don't need. If you never intend to make prints at all, but just put photos on the web, a very modest camera with a good feature set is what you want.
Tom's Photography Info / firstname.lastname@example.org