About Censors and Megapixels
The electronic censor has replaced the film of yore in digital cameras. Although other forms of censors are also present, digital cameras have either a CCD or a CMOS chip as a censor. CCD stands for 'Charge-coupled Device. CMOS stands for 'Complementary Metal Oxide Semiconductor'. The debate whether one is superior to the other is pointless for our purpose. Suffice it to say that both will yield satisfactory results and you are more likely to find a CCD in a consumer camera.
The censor consists of thousands of picture cells, or pixels, that gather light from a scene and pass it to a storage device, before moving on the next scene, quite like the eye.
Unfortunately, the caliber of a camera has come to be judged by this single yardstick that counts the number of thousands of pixels, or megapixels, that the censor contains, abetted by the manufacturers. Nothing could be farther from truth.
The picture quality of a camera depends on a combination of factors like the composition of the lens ( a lens consists of several optical elements), its metering and focusing system, the filters and censor used and several complex processing algorithms. A compromise in any of these vital parts will ruin the image. If the digital cameras 5 to 7 years back struggled for a decent picture output, it was not because of low megapixel count alone. As the technology matured, the censor as well as every other component could yield better output. The pixel count in the censor kept creeping up as part of the evolving process. However, once this count crossed the threshold of 6 megapixels, any further rise stopped having any positive bearing on the image, particularly in case of compact cameras. Lets see why that is the case.
The compact cameras have tiny censors which consist of pixels. Imagine these pixels as containers that collect light. As the manufacturers keep cramming up more and more of them on the same tiny chip, their size keeps getting smaller and smaller and accordingly, their light storing capacity gets reduced. While the rise in pixel count in DSLR cameras, which have huge censors anyway, gets complimented by sophisticated, fast lenses (each of which cost thousands of dollars alone), the constraint imposed by the tiny format of the compact cameras, which have baby sized lenses, rules out similar spectacular performance. Thus, the lack of light, or the lack of signal, has to be compensated by boosting the sensitivity of the pixels which results in a lot of electrical noise, which translates into lot of alien coloured grains in the image. A lot pf processing is involved in segregating the signal from the noise and the image does loose a lot of relevant information in the end. So don't be shocked if a 5 megapixels camera turns up a cleaner imager with perfect tones and natural colours than an 8 megapixel camera. In short, if the manufacturers get honest about improving the resolution of the censor, they would increase its size to suit the increase in numbers, which would necessitate increase in the size of the lens and all this would cost them a lot of money. Why would they do this when they have successfully sold the myth of megapixels alone to the unsuspecting masses?
If , however, you are convinced by the logic that more megapixels allow you to crop an area of the image and since the cropped area will still have enough information to get a poster developed, you seriously need to
(1) improve your composing technique;
(2) look for a DSLR camera with a combination of good, fast lenses.
As for as being able to differentiate the output of a 5 megapixel, an 8 megapixel and a 13 megapixel camera, do go through an interesting experiment by David Pogue of New York Times.
Also, Ken Rockwell has a lot to say on the subject.
Thus, the ever rising count of megapixels is nothing but a ploy the manufacturers use to sell you newer models.