At the heart of every digital camera is a light-sensitive silicon chip called a sensor. The sensor takes the place of film for recording images.
The size of these rectangular chips varies depending on the type of camera. In small compact cameras the sensors are relatively tiny, sometimes no larger than the size of the fingernail on someone's little finger.
Above: The full-frame sensor in the Nikon D700
The imperial measurements used to indicate some sensor's sizes are a hangover from the old days of television camera tubes. They give diagonal measurements as fractions of an inch and many manufacturers do not publish exact dimensions for the sensors in their compact cameras.
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It can be very confusing to understand and is best explained by a diagram showing the various relative sizes of different types of sensors.
Above: The relative size of sensors from the smallest type used in a compact camera all the way up to a full-frame digital SLR
Sensors are packed with up to 24 million light-sensitive points or pixels that can register the amount of light falling on that part of the silicon. A very small microlens sits above each pixel and converges light rays from the camera lens on to the pixel below.
Each pixel generates a very small electrical charge when hit by light, which is then amplified and turned into a digital signal that's used alongside the signals from all the other pixels to create an image. It's a similar principle to the light-collecting rods and cones in the retina of a human eye.
You can think of the sensor as a mosaic of light-sensitive points that create a photo. Obviously, the more pixels (or resolution) there are on a chip, the more detailed the image can be. As with a mosaic, smaller tiles make a more detailed image; that's why a 6-megapixel sensor produces a less-detailed image than, say, a 12megapixel sensor… because it's made up of fewer, larger points.
Large and small
Most compact system cameras (CSCs) and digital SLRs use larger sensors than the average compact camera.
A good rule of thumb is that the larger the size of the sensor, the bigger each light-collecting pixel can be and the more light it can record without the need to have the signal over amplified. The result of these bigger pixels is a lovely sharp, clean image that doesn't suffer from graininess when setting the camera at higher ISO or sensitivity levels.
At top end of the market, digital SLRS have either APS-C (or DX-sized sensors) or full-frame chips, which are the same size as the image area on an old-school piece of 35mm film. These really large sensors have both a high resolution and large pixels, which means they can record high-quality images even at high ISO sensitivities. These sort of images can be reproduced at very large sizes while maintaining really high quality.