The sensor in a digital camera is the electronic version of film, and records light to create a digital image. The size of the sensor determines how much light can be recorded after travelling through the lens of the camera, with a large sensor receiving more light, and a smaller sensor much less. The more light the sensor can record, the better the image quality. The technical aspects of sensor types and designs will be looked at in another article, but here we'll look at the effect sensor size has on image quality.
The most commonly used sensor sizes are Full frame (DSLRs), APS-C ("Crop sensor" Canon, Nikon, other DSLRs), Micro Four Thirds (Olympus, Panasonic), 1inch / CX (in Nikon 1 cameras), 1/1.7inch (in "Serious compacts") and 1/2.33inch in compact cameras, although there are occasionally other sizes used, for example in some compact cameras, and even smaller sensors (typically 1/3.2inch) can be found in camera phones.
Nikon 1 S1 (1inch), Olympus OM-D E-M10 (Micro Four Thirds), Samsung NX30 (APS-C), Sony Alpha A7 (Full-frame) - click to enlarge.
Shown above is a relative diagram showing the size of many of the common sensors found in digital cameras, with the full frame sensor equalling the size of a 35mm film. Mobile phones often use a smaller 1/3.2inch sensor, as do many digital camcorders.
The crop factor is used to calculate what the effective focal length of a lens would be in 35mm terms based on the focal length printed on the lens. For example, if you put an 18-55mm lens on a Canon EOS 600D with a 1.6x crop factor, you get the equivalent of a 28-88mm lens in 35mm terms.
If you've ever wondered why sensor size is referred to as a fraction of an inch, then this is one of the best explanations around, and a much more useful indication would in fact be using the crop factor, or pixel size instead.
Noise results from different size sensors:
Shown here are examples of what you can expect to see in regards to image quality and noise performance from different sensor sizes, each photo has been taken at ISO3200. Things to look out for include loss of colour as the ISO setting increases, loss of detail and edge quality, as well as any random colours appearing. We have picked a variety of 12 megapixel cameras where possible, with the exception of the 1inch sensor in the Nikon J1/V1, as this is a 10 megapixel sensor. Shown below are 100% crops, showing the actual pixels of images taken with each camera.
As you can see, for the most part, as the sensor size decreases, the noise levels increase due to the size of each pixel decreasing on the sensor, and this in turn means the camera can't record as much light. Where this can change is where companies use different noise reduction processes in the camera, with Fujifilm as an example having some of the best noise reduction procedures.
Find a more in-depth look at sensor and pixel size here.
Find out all about digital camera sensor technology.