Digital cameras use image sensors of different sizes. A smaller sensor records a smaller part of the image that the lens projects, so it gives a narrower field of view with the same lens. The crop factor compares a sensor to a full-frame sensor (36 x 24mm).
To find the full-frame equivalent field of view, multiply the focal length by the crop factor. APS-C cameras have a crop factor of about 1.5x (1.6x for Canon), so a 50mm lens frames like a 75mm or 80mm lens. Micro Four Thirds cameras have a crop factor of 2x. Full-frame cameras have a crop factor of 1. The real focal length of the lens does not change.
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