Crop Factor & Equivalent Focal Length
Crop factor from the sensor diagonal, the full-frame equivalent focal length and depth-of-field aperture, and the lens that frames the same on another format.
The crop factor against full frame, the equivalent focal length and the aperture that gives the same depth of field, and the focal length and aperture that match your framing on any other sensor format.
Example: 35 mm f/1.8 on 23.5 × 15.6 mm APS-C has a crop factor of 1.53: it frames like 53.7 mm on full frame, with the depth of field of f/2.76.
Ratio of
diagonals.
What crop factor measures, how it gives equivalent focal length and aperture, how any two formats compare, and what equivalence does not change.
Crop factor
Crop factor is the full-frame diagonal, 43.27 mm, divided by a sensor’s diagonal. A smaller sensor records a smaller part of the image circle, so a lens on it frames like a lens crop × longer on full frame.
Equivalent aperture
At the same framing and viewing size, depth of field scales with the f-number times the crop factor: f/1.8 on APS-C gives the depth of field of about f/2.8 on full frame. Between any two formats the ratio of their diagonals does the same job for focal length and aperture.
Aspect ratio
Crop factor matches diagonals. A 4:3 sensor compared with a 3:2 one frames the diagonal the same but the width and height differently; the page warns when the aspect ratios differ.
What the number does not settle
Exposure does not change: f/2.8 gives the same brightness on every sensor. Noise, dynamic range and lens sharpness are outside the arithmetic. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.