Crop Factor Calculator

The formula

The crop factor (also called the focal-length multiplier) is the ratio of the full-frame 35mm sensor diagonal (≈ 43.3mm) to your sensor’s diagonal. Two simple multiplications convert any lens to its full-frame equivalent:

equivalent focal length = focal length × crop factor
equivalent aperture     = aperture × crop factor

The focal-length multiplier changes the angle of view (how much fits in the frame). The aperture multiplier is the equivalent aperture for depth of field and total light gathered, which matters when you compare background blur or low-light reach between systems. The number printed on the lens barrel, for exposure metering, never changes; an f/1.8 lens still meters as f/1.8.

Worked example

A 50mm f/1.8 lens on an APS-C body with a 1.5 crop factor:

• equivalent focal length = 50 × 1.5 = 75mm
• equivalent aperture = 1.8 × 1.5 ≈ f/2.7

So it frames like a 75mm portrait lens on full frame and renders depth of field roughly like an f/2.7 lens would, still lovely, just a touch less blur than the f/1.8 marking suggests.

Common crop factors

Pitfalls to avoid

• Do not adjust exposure for the equivalent aperture. Use the marked f-number for metering; the equivalent aperture is only for comparing depth of field and total light between formats.
• Do not apply crop factor twice. If a manufacturer already lists an “equivalent” focal length, it is full-frame already, multiplying again is wrong.
• ISO behaves like aperture. To match total light, a larger crop factor usually means raising ISO, which is why bigger sensors tend to look cleaner at the same equivalent settings.