Something I have wanted to cover in my Photography Class that I just haven't had time to do is the myth of the megapixel when purchasing cameras. When people ask me about my cameras the first question that inevitably comes up is "How many megapixels" it has. Somehow camera manufacturers have tricked the general public into thinking that more megapixels in a camera equates to a better image. Unfortunately this is far from the truth.
Allow me to describe what is happening in a digital camera so I can explain why "more megapixels" is probably a bad thing. Digital cameras work by focusing an image onto an analog image sensor that converts light into electrical impulses (similar to the way the human eye works) in millions of tiny sites called pixels. These signals are processed by another chip and are converted into a digital image that is stored on a memory card. Unfortunately this process isn't perfect, and has a few problems. One of those problems is electrical noise in the image sensor itself, and that electrical noise shows up as random variations in color and brightness in individual pixels in your final picture.
In my first class we talked about ISO, which in a digital camera is effectively the sensitivity of the image sensor. One might think that more sensitive is better, and in theory this may be true in certain circumstances. However, to accomplish higher sensitivity, the camera uses amplification of the signal coming off of the sensor, and the result is in the random noise I just mentioned. However, the amount of electrical noise in the chip is more or less constant no matter how much light comes into the camera, so by dialing up the sensitivity you get more noise and less signal from the light entering the camera. The ratio of actual signal to electrical noise decreases as you turn up the sensitivity; less signal to a constant amount of noise. High ISO settings result in noisier pictures.
Another way to increase sensitivity of a chip is to make the pixels larger so they can capture more light. Since the electrical noise in the chip remains more or less constant, more light means a cleaner image. But to make larger pixels means you either have to make the chip larger, or cut down on the number of pixels in a given area. But the trend among camera manufacturers is going the other direction… stuffing more pixels into the same size chip (larger chips are considerably more expensive to manufacture and require larger lenses, also very expensive to make). The result? A smaller surface to capture light, which means an increasing amount of noise in the pictures we are getting out of newer cameras. And the higher levels of noise mean that the camera has to do more work to try to remove that noise, and removing noise also means removing real image detail; the camera can't discern between the two. So your final output ends up being a low resolution noisy mess.
More megapixels would be fine if the sizes of the chips and lenses were increasing. But there is another disturbing trend going on there; the lenses and chips that cameras are equipped with is actually decreasing rather than increasing because the cameras themselves are getting smaller and smaller. Not a good thing. The level of noise has gotten so bad that the high ISO settings on most new cameras are basically unusable.
So in terms of raw specifications and the reality of what they mean, the newer digital cameras that are coming out now are actually inferior to their predecessors in more than one way. The Canon 40D that I just bought, for example, though it has a 10.1 megapixel sensor actually produces a visibly noisier image (at high ISOs) than the 20D that it replaces with its 8.2 megapixel sensor. (Fortunately though, because of the large sensor used in both cameras the level of noise is so low, or at low ISOs completely invisible, that I don't mind the tradeoff between additional noise and new features. The same can't be said of compact point-and-shoot digital cameras, however.)
If megapixels aren't the best indicator of the ultimate quality of the image coming out of a camera, what is? Turns out, it's the size of the lens. I'm talking about the glass itself, not the barrel surrounding it. Larger lenses let in more light, and lenses are generally matched to the size of the sensor behind them; larger lens usually means larger sensor. So when comparing two cameras with generally equal specifications, the one with the larger lens is usually going to produce a much better image. (It shouldn't be any surprise, therefore, that cameras with nice big lenses also cost more.)
So when shopping for your next digital camera, don't be swayed by the "megapixel" number on the specification chart of the camera. If you fall for that trap you may end up with subpar pictures and end up paying more money for the privilege.
P.S. One of these days I might discuss how all camera manufacturers are lying to you about the number of megapixels anyway, overestimating by three times the actual number, but I'll save that for another day.