Crop factor is the ratio between a full-frame sensor's size and a smaller sensor's size — typically around 1.5-1.6× for APS-C or 2× for Micro Four Thirds — used to convert a lens's focal length to its full-frame-equivalent field of view.
Crop factor is the number that lets photographers compare lenses across cameras with different sensor sizes. A lens projects the same image circle whatever body is behind it, but a smaller sensor only captures the middle of that circle, so the field of view is narrower and the shot looks more zoomed in. Multiply the lens's focal length by the crop factor, roughly 1.5 for most APS-C and 2.0 for Micro Four Thirds, and you get the full-frame focal length that would frame the same scene.
Ignoring this leads to real mistakes: someone who buys a 24mm lens expecting a wide interior shot on an APS-C body gets a 36mm-equivalent view instead, and half the room falls outside the frame. It also works in your favour for wildlife, where a 400mm lens on a crop body reaches like a 600mm on full frame at no extra weight or cost.
A wedding photographer used to full frame picks up an APS-C body as a second camera and mounts a familiar 35mm lens, expecting the same moderately wide look. With the 1.5 crop factor, that lens now frames like a 52mm on full frame, tight enough that group shots at the reception push the photographer back against the wall. Switching to a 23mm lens on the crop body restores the 35mm-equivalent view.
The same maths helps at the long end: a 70-200mm zoom on that APS-C body covers a 105-300mm-equivalent range, genuinely useful reach for candid ceremony shots from the back of the room. Depth of field shifts too, since matching the full-frame framing from one spot means the crop sensor is effectively working about a stop deeper at the same f-number.
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