When I travel on assignment, I often have several digital cameras available for my use. They may vary from a compact "Point & Shoot," to a relatively massive professional model, such as the Canon EOS-1Ds Mark III. No matter which I use, someone inevitably asks "How many pixels does that have?" I have learned to just answer the popular question directly and simply, even though that information is of limited use to most casual photographers.
How Pixels Work in a Camera
Very simply put: a pixel is a very small (essentially impossible to discern with the naked eye) rectangle of material that changes its characteristics when briefly exposed to light. When millions of them are applied to a flat rigid surface, they become the "film" in a digital camera. When light is reflected from a subject, passes through the camera's lens, and strikes that "film" (let's call it by a more correct name: "sensor"), a latent image is created, in which each pixel temporarily records and stores a record of the quality of light having hit it. Almost immediately, the camera's built-in computer reads that information, clears the sensor so that it is ready to accept another image, and then processes the data, creating a photo file, so that the image can be viewed on the camera's LCD screen, and saved for further future use.
Not All Pixels Are the Same
The higher the quality of each pixel, the better can be the quality of images created thereon. Two, of several relevant factors, have significant impact on the quality of pixels. One is the type of materials used to make the pixels, and the care with which that process is carried out: the other has to do with the size of each pixel. In this discussion, we'll assume that quality of materials and construction are alike, focusing only on the numbers of pixels on a sensor, and the size of each of those individual pieces. The larger the pixel, the better able it is to record data when struck by light. Smaller pixels are less sensitive to light, so amplification must be raised when reading the weaker signals. That amplification results in digital "noise" (e.g., graininess and mottled specks) which is especially noticeable when the camera is set at higher ISO values, such as 800, rather than the usual 100. In addition, smaller pixels are not well able to record the wide range of illumination seen in bright sunlight and deep shadows.
Sensor Sizes Vary
Because the size of sensors in digital cameras (especially those used by travel photographers and journalists) varies from around 5.76x4.29mm, to as much as 35.8x23.9mm, you can see that if there are 8 million pixels (8 MP) on both of those sizes, each pixel can be, and is, much larger when on the larger sensor.
The Bottom Line
Camera manufacturers and sellers have sensitized us to pixel count as some sort of categorical measure of a digital camera's capabilities; that is wrong! While an 8MP point & shoot, pocketable camera may be capable of making many photos suitable for such as illustration on a Web site, an 8MP sensor in something such as the Canon Rebel series, or Sony D-SLRs, will, under similar conditions, make significantly superior images. Indeed, a 10MP sensor, in something such as a Canon EOS-1Ds (which has the largest available CMOS sensor) is capable of making photos dramatically better than those made similarly, but using a 20MP sensor in a pocketable camera (if such was possible). So you see, the number of pixels isn't as important as you might have thought it was.
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