A typical digital camera captures image information using image sensors, the digital equivalent of the film. Exposure factor notwithstanding, the quality of image captured depends on a few factors such as the sensor size, type and pixel density on the sensor. We'll discuss these few factors without going too deep into technical details to give you a better understanding on how image sensors work, and why more megapixels is not always a good thing.
Types of sensors
There are 2 major types of image sensor for digital cameras and camcorders: CCD (charged couple device) and CMOS (complimentary metal oxide semiconductor, or sometimes known as active pixel sensor). There have been heated debates of which sensor technology is more superior than the others, but we're not going to go there. Suffice it to say, CCD and CMOS works in very different way. High end DSLRs cameras use CMOS sensors because they are easier to make in bigger sizes, and consume less power, but they cost more. On the other hand, compact cameras and digital camcorders commonly use CCD sensors. The newer generation of Nikon DSLRs starting with the D90, D300 and all the way up to D3x use CMOS sensors, much to the delight of photographers.
The main thing about sensors is really about the size. A picture taken with the latest 5 megapixel camera phone will never look as good as the entry level DSLR by a good mile (let's not talk about lens quality first). This is due to the huge difference in sensor size used on both devices. The chart on the right compares the typical sensor sizes found in common digital cameras today. The sensor on a compact camera is approximately 10% the size of a typical DSLR , and a full frame DSLR has a sensor 20 times bigger! This is also the main reason why a DSLR cost upwards of RM3000 while a compact can cost as little as RM500. A 'full frame' sensor size is the equivalent of a standard film, at 36mm X 24mm. So the bigger the sensor, the better the image quality you'll get. That's fact number 1. Another fact to consider, before you plunge you hard earned cash on that latest digital camera is how many megapixels it shoots relative to the size of it sensors. This is what pixel density is all about.
Pixel density and Signal/Noise ratio
Let's say we have a fixed sensor size of a typical compact camera. As the megapixel war between camera manufacturers escalate, they shove more and more megapixels into these cameras without enlarging the sensor size much because larger sensors are more expensive to make. More megapixels on a same sensor size means smaller pixels are crammed into the sensor, as example below indicates. Small pixels are bad when it comes to translating light into digital bits of data. It becomes less sensitive to light and you'll get more digital noise (the rainbow coloured grain you see when you shoot in low light conditions).
To understand how sensors work, imagine each pixel on the sensor as a bucket. Imagine light falling into the bucket as coloured balls. Not all light that falls onto the pixel sensor (buckets) is translated as information used to create the final image. The ratio between the good (signal) and the bad (noise) determines how well the image is captured, this is called the signal to noise ratio. So the smaller the pixel/bucket, the lower the signal/noise ratio. Bigger buckets (pixels) will produce higher signal/noise ratio which will result in better photos especially in low light condition.
Nikon's top of the line D3 is a full frame shoots at 12.2 megapixels, while its competitor produces 23 megapixels with the same sensor size. This is why the D3 has won raving reviews and praises from professionals with its superb ability to capture low light photography with very little noise. With the introduction of D3X, the jury's still out on whether the increased pixel count on the same sensor size will produce similarly exceptional quality photos like the D3.
So the next time you're out buying a new digital camera for yourself, the wife or mistress, don't be fooled by the salesperson flogging the latest compact cameras with 20 megapixels capability. More megapixels is not necessarily a good thing. Remember this: we want bigger pixels, not more pixels.