Crop Factor
The ratio between a sensor's diagonal and that of full frame, used to convert focal lengths into their full-frame equivalents.
Formula / rule
Crop factor = 43.3 mm / sensor diagonal
Measured in
× multiplier
In depth
Crop factor changes nothing about the lens: a 50 mm lens still has 50 mm of focal length and still forms the same image circle. What changes is how much of that circle the sensor records. Applied consistently, the same factor also converts aperture for depth-of-field and total-light comparisons, which is the part most equivalence arguments get wrong.
Examples in practice
A 1.5× crop factor makes a 35 mm lens frame like a 52 mm lens, and its f/2 render depth of field like f/3 on full frame.