The sensor in a Canon 1000D (Digital Rebel XS) is 22.2 x 14.8 mm is size and contains more than 10 million pixels in a 3888 x 2592 pixel array.

The sensor in a DSLR camera is made up of a grid, or array, of pixels like the squares in a checkerboard.

The Canon EOS 1000D (Digital Rebel XS), for example, has an array of 3888 x 2592 pixels in a sensor that has a physical size of 22.2 x 14.8 millimeters. This roughly 22 x 15mm sensor size is also known as APS-C size, a name carry-over from the days of film where APS meant Advanced Photo System. This was a smaller size that was supposed to replace traditional 35mm-sized film which had dimensions of 36mm x 24mm. A 36 x 24mm sensor in a DSLR is called a full-frame sensor.

A sensor array of 3888 x 2592 pixels has 10,077,696 pixels in it. This is roughly 10 million pixels. In computer-speak, we use the prefix "mega" to refer to a million, so we say the Canon 1000D has a 10 mega-pixel sensor. Some cameras, like Canon's 60D have 18 mega-pixel sensor.

Beware the Hype!

Don't get caught up in the mega-pixel hype that is put out by the marketing departments of the camera manufacturers. Cameras have now reached the point where increasing the number of pixels in a sensor does virtually nothing to improve the quality of the images it produces. In fact, the dirty little secret that they don't want you to know is that few of their lenses are capable of taking advantage of the increased resolution that these high pixel-count sensors have.

In practical, every day, or night, use, you won't be able to see much improvement, if any at all, between a 10 mega-pixel sensor and a 12 mega-pixel sensor.

With higher pixel-count sensors, such as a 18 mega-pixel camera, you will also have more demanding space requirements for archiving your images, as well as the need for more RAM and processing power in your computer. Higher-resolution sensors also require better optics.

The Pixel Grid in Numbers

Photons that are recorded by the sensor are eventually turned into numbers. That is all a digital image file is - a collection of numbers. Each pixel is represented by three numbers - the brightness value of the red, green and blue channels that make up the color in the image.

A digitized image is made up of a grid of pixels which are represented by numbers. The numbers specify the pixel's location in the grid, and the brightness of the red, green and blue color channels. In the image above at far right, the arrowed pixel has three numbers associated with it: 252R, 231G, 217B. This means that the pixel has red, green and blue brightness values of 252 for red, 231 for green and 217 for blue. The numbers are for a scale that runs from black at 0 to pure white at 255.

Normally, these numbers run from 0 to 255 with 0 representing black, and 255 representing white in an 8-bit-per-channel file. Each individual number between 0 and 255 represents a step of brightness from black to white. You may not think 256 steps is a lot, but when you combine all three channels, you multiply each, and you end up with more than 16 million possible combinations, where each combination represents a unique color, in a 24-bit color file. A color file is called 24-bit color because it is made up of three channels (red, green and blue), each of which has 8 bits of tonal detail in it.

When the color of an individual pixel is discussed, it is usually given by its red, green and blue pixel numbers, such as 90R,120G,75B, which is the color of a clear blue sky. Usually the numbers are read in the order of red, green and blue, so the letters are left off after the numbers, such as 90,120,75.

Under this system, a pixel with RGB numbers, or values, of 0,0,0 would be black. 255,255,255 would be white. 255,0,0 would be pure red. 0,255,0 would be pure green. 0,0,255 would be pure blue. Any pixel with the same values in all channels, such as 128,128,128, would be a given brightness of gray.

Sensors and Pixels - The Bottom Line A DSLR sensor is comprised of millions of individual pixels which record photons of light. You probably won't be able to notice the difference between a camera with 10 million pixels and 12, or even 15 million pixels. Don't worry about the megapixel count in a camera when you buy one. Any recent model DSLR with Live View will be good for long-exposure astrophotography.