Darren Crush explains
In terms of today's DSLRs, most sensors are the APS-C-sized format, smaller than that of a 35mm (ie full) frame. What this results in is a magnification of a lens's focal length, whose value is determined by the sensor's size in comparison to a standard full frame. Manufacturers refer to this magnification as a ‘crop factor' - or, how much of the image is cropped due to the smaller sensor.
The APS-C format, measuring around 24x16mm, is smaller than a full-frame sensor by a factor of 1.5x. So, the focal length of a lens must also be multiplied by this amount to arrive at its effective (35mm equivalent)focal length - so a 28mm lens becomes a 42mm lens, a 50mm lens become a 75mm lens and so on. The APS-C format does vary slightly according to different manufacturers, with Canon's APS-C sensor at a factor of 1.6x and Sigma's at 1.7x, but in any case it's simply this figure that needs to be multiplied by the focal length to arrive at its effective length.
The most obvious way manufacturers have accounted for this is by adapting their lenses. As the traditional wideangle lens used to have a focal length of around 28mm, most kit lenses start from 18mm to meet this length (ie, 18 x 1.5 = 27mm). This is equally the case for the Four Thirds format, whose 2x multiplication factor means that Olympus's kit lenses start at 14mm. As only the central part of the image is used by the sensor, it allows digital lenses to be both lighter and smaller, as less glass is needed in their construction.
Full-frame 35mm sensor
A comparison of the different-sized sensors. While the full-frame format is fixed at 36x24mm, the APS-C format comes in slightly different sizes.