Photography Tech Tip : Sensors

This is one in a series of 'Tech Tip' articles intended as a guide to digital camera technology and terminology. Its aim is to demystify some of the language used on this site to allow you to make more informed decisions when considering purchasing a new digital camera.

SENSORS
Whilst much emphasis is placed on digital camera resolution, every megapixel is not created equal. The sizes of sensors within digital cameras vary considerably, with typical sizes including:

• 1/2.5-inch (5.8x4.3mm)
• 2/3-inch (8.8x6.6mm)
• 'APS-C sized' (22.7x15.1mm)
• '35mm full frame' (36x24mm)

The first two are typically found in compact, point-and-shoot cameras; the second two are more commonly found in digital SLR type cameras. The relative sizes of these sensors can be seen in the image below:

The range of resolution is approximately the same across both compact- and SLR-type digital cameras, yet a 35mm full-frame sensor has almost 35x the area of the smallest compact sensors. It would be possible to create a camera of 100 megapixels or more using the smaller sensors and the same area, so why not do that?

The answer is one of quality. For two sensors of equal resolution, each 'photosite' (the area responsible for one pixel of image) is larger on the larger sensor, which allows more light to enter. With more light available to it, each photosite is able to record more accurate tone and colour detail. Furthermore, because the amount of light entering the photosite is increased, the susceptability to interference in the form of 'noise' is reduced. In technical terms, the signal-to-noise ratio (SNR) is improved (see Tech Tip: ISO and noise).

When comparing cameras of the same resolution, always compare the sensor size, too; a larger sensor should provide higher-quality images. Equally, when comparing cameras of different resolutions (particularly when they both exceed your requirements - see Tech Tips: Megapixels), also take the sensor size into account as the extra resolution may be to the detriment of quality.

For compact cameras, sensors are often quoted as fractions of an inch, e.g. 1/2.5 inches or 1/2.33 inches. For digital SLRs, named standards such as 'APS-C' (22.7x15.1mm) or '35mm full-frame' (36x24mm) are common. This information is often quoted with the resolution in the manufacturers tech specs.