Sensor crop and focal-length equivalence
Equivalent focal length describes framing relative to the reference format. The physical focal length engraved on the lens does not change when the sensor changes. Different aspect ratios can make horizontal and vertical field-of-view comparisons differ from the diagonal comparison.
For checking the shape of an image after selecting its framing, the Image Aspect Ratio Calculator compares its width and height.
The formula
The 36 by 24 mm format supplies a diagonal reference. Comparing sensor diagonals gives a field-of-view equivalence factor. Dividing a desired equivalent focal length by that factor gives the physical lens focal length for the smaller or larger format.
Worked example
A 24 by 16 mm sensor has a diagonal crop factor of 1.5 relative to 36 by 24 mm. A 50 mm lens has a 75 mm equivalent diagonal field of view. To obtain that 75 mm equivalent, the needed physical lens focal length is 50 mm.
How to use this calculator
- Enter sensor width, sensor height using the units shown.
- Set actual lens focal length, desired 35 mm equivalent focal length.
- Calculate and review diagonal crop factor alongside the supporting quantities.
Use the active image area
Enter the sensor area actually used for the image. Video crops, stabilization, or a camera crop mode may use less than the full sensor dimensions. Manufacturer sensor labels are not always literal millimeter dimensions, so use the specified active width and height. The diagonal factor is especially useful when the compared formats share the same aspect ratio.
Equivalence does not change optical focal length
Mounting a lens on a smaller sensor records a narrower portion of its image circle; it does not turn the lens into a physically longer one. Exposure f-number also stays unchanged. Perspective depends on viewpoint, while depth of field comparisons need aperture and framing considerations beyond this field-of-view calculation. Lens coverage and compatibility must be checked separately.
Assumptions & limitations
What this calculation assumes
- A 36 by 24 mm reference and diagonal field-of-view comparison are used.
What to keep in mind
- Excludes perspective changes, aperture equivalence, lens distortion, and mount compatibility.
Common questions
Does a 50 mm lens become a 75 mm lens?
No. Its focal length remains 50 mm. The smaller format records framing comparable to 75 mm on the reference format.
What if my sensor has a different aspect ratio?
The result matches diagonal framing. Horizontal and vertical comparisons can differ because the image shapes differ.
Does crop factor change the marked f-number?
No. The exposure f-number remains the lens setting. This page compares diagonal framing only.
Sources & further reading
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