For one reason or another, I tend to be regarded in certain circles as the local photography expert. People come up to me to ask for advice, which is usually just their way to convince themselves that the choice they already made was good rather than truly get my opinion (although I have to say that there is rarely a truly bad product out there and my own personal preferences are in no way better than their choices).
Anyway, I'm always surprised by the importance people attribute to mega pixels. I have to hand it to camera manufacturers, they really managed to get us all to judge a camera by this one number (usually a single digit) and one number alone. It's so brilliant when you think about it: all they need to do in order to sell you their next model is marginally increase that magic number. It could backfire on them, though: a friend of mine said a while ago that once mobile phones have 15mp cameras built in no one would ever need to buy any other photography equipment.
Just imagine what would happen if we were to judge cars by a single number, say horse power. Tanks would rule on high! Or if we were to judge people solely by their weight: We'd all be eating all day long.
Anyway, to make camera issues clearer and help the world get rid of crap myths, here is the official MR list of things that do determine a photo's quality. I will totally disregard technical stuff such as camera reaction time. Enjoy:
1. Light handling:
A digital camera is a device that captures analog light and stores a digital representation of it. It therefore needs a mechanism to enable it to handle that analog light, and that is the thing we commonly refer to as a "lens". Contrary to common belief, lenses are not all equal even if they have the same zoom specifications, just like a cheap $100 speaker rated to handle 250 watts RMS will not sound the same as a $75000 Wilson Grand Slamm speaker with the same rating.
Lenses are where the above friend's theory on mobile phones would fail, by the way, because no one would be able to create a small lens to fit in someone's pocket that would be the equal of a larger lens. His opinion that instead of using a zoom lens you would just crop a high mega pixel photo does not apply, too, simply because the lens' setting does not determine the zoom alone but also the perspective (the relation between different objects): that is why people's faces appear distorted in a wide angle photo - things closer to the camera appear bigger than they are - and that is why we can have the illusion of someone holding an elephant on the palm of his hand by using a zoom lens that pushes everything close together.
2. Digital processor quality:
Every digital camera needs an analog to digital converter to decode the analog light it receives through the lens to a series of numbers (aka digital format). Again, not all sensors are created equal; the famous mega pixel count is just one of the sensors' attributes, but did you know that for a given sensor size, the more mega pixels it has the more noise the photo will have? More mega pixels do not, therefore, necessarily mean a better photo.
If we take your common 6mp sensor, it would be made of roughly 2mp of red sensing cells, 2mp of blue, and 2mp of green (actually, green would have a larger share, because our eyes are more sensitive to it). Each one of those cells would be able to tell between 256 levels of light in their own specific color, and the photo you end up seeing is a mathematical extrapolation of the output of all cells combined. Obviously, it doesn't take a genius to realize there is plenty of room for improvement in such a mechanism, regardless of mega pixels. Which gets me to the third and last point-
3. Digital processing:
The way the camera processes this data its sensor provides is important, too. Each manufacturer has its own proprietary algorithm for doing that: determining the color of a red pixel surrounded by a combination of blue and green pixels and the levels of each. Today you don't even need to settle with what your camera does: If your camera supports the RAW format, which stores the direct and detailed output of the sensor, you can let your desktop do the processing for you (that P4 monster at home is much more capable than your pocket size camera, and Photoshop is way too big to be contained in a camera).
Hope that settles it. The next time someone tells you that his mega pixel count is bigger than yours, tell him/her it's not the size that matters.
(Somehow, I can now see why the camera manufacturers are so successful with their marketing techniques)