Filming & Production
Crop factor
Also called: equivalent focal length, focal length multiplier
Put a 50mm lens on a full-frame body and on an APS-C body, and the smaller sensor shows less of the scene because it sits inside the image the lens projects. Crop factor is how the two get compared: roughly 1.5x on most APS-C sensors, 1.6x on Canon's, 2x on Micro Four Thirds. Multiply and you get the full-frame equivalent, so 50mm on Micro Four Thirds frames like a 100mm lens.
The lens itself has not changed. Focal length, compression and perspective stay exactly as they were; the sensor is recording a smaller part of what arrives. For a desk setup it matters because a lens that frames you comfortably on full frame can be too tight to fit you in on a crop body.