Digital SLR sensors come in a number of different sizes, but the most common is the APS-C format. This is about half the area of a full-frame sensor or a 35mm film frame, and gets its name from being about the same size as the APS-C compact film cameras that were popular just before the digital revolution.

This diagram shows the relative size of APS-C sensors and full-frame sensors. APS-C sensors can deliver very good image quality, but the extra size of full-frame sensors means that they will always be better still, and for professional photographers this will be important.

The diagram also illustrates why you have to apply a ‘crop factor’ or ‘focal factor’ (1.5x or 1.6x, depending on the exact size of the sensor) when working out focal lengths with APS-C format digital SLRs. For any given lens, they capture a smaller angle of view than a full-frame sensor, which gives the effect of shooting with a longer focal length.