Lens Focal Length Comparison Calculator
Compare how two lens-and-sensor combinations deliver equivalent fields of view. Use it when switching camera systems or renting a lens to match your existing look.
About this calculator
Different sensor formats capture different fractions of the image circle projected by a lens. A full-frame sensor (36 mm) sees the widest view; smaller sensors crop it, effectively multiplying the focal length. The equivalence ratio is calculated as: ratio = (originalFocal / originalSensor) × (targetSensor / targetFocal). A ratio of 1.0 means both setups produce an identical angle of view. Values above 1 mean the target combo is telephoto-equivalent; values below 1 mean it is wider. Common sensor diagonal values used as proxies are 43.3 mm (full frame), 28.4 mm (APS-C Nikon), 26.7 mm (APS-C Canon), and 21.6 mm (Micro Four Thirds). Understanding this lets photographers replicate a specific look when changing bodies.
How to use
You own a 50 mm lens on a full-frame body (sensor = 43.3) and want to know what focal length gives the same field of view on a Micro Four Thirds body (sensor = 21.6). ratio = (50 / 43.3) × (21.6 / targetFocal). Set ratio = 1 and solve: targetFocal = (50 / 43.3) × 21.6 ≈ 24.9 mm. So a 25 mm lens on Micro Four Thirds matches the perspective of a 50 mm lens on full frame. Enter your own values to compare any pairing instantly.
Frequently asked questions
What is crop factor and how does it relate to focal length equivalence?
Crop factor is the ratio of a full-frame sensor diagonal (43.3 mm) to your sensor's diagonal. An APS-C sensor with a 1.5× crop factor makes a 35 mm lens behave like a 52.5 mm lens on full frame. This calculator generalises crop factor to any two sensor formats, not just full-frame comparisons. It lets you find equivalent focal lengths between APS-C, Micro Four Thirds, medium format, or any custom format.
Why does sensor size affect the field of view of a lens?
A lens projects a circular image onto the camera. A larger sensor captures more of that circle, showing a wider scene. A smaller sensor captures only the centre portion, cropping out the edges and making distant subjects appear closer — the telephoto effect. The focal length of the lens itself does not change, but the captured angle of view shrinks proportionally with sensor size. This is why a 50 mm lens feels like a portrait lens on APS-C but a standard lens on full frame.
How do I match my full-frame lens look when switching to a crop sensor camera?
Divide your full-frame focal length by the crop factor of the new system to find the equivalent native focal length. For example, moving from full frame to a 1.5× APS-C body means a 35 mm lens on full frame is replicated by approximately a 23 mm lens on APS-C. Use this calculator to check any combination by entering both sensor formats and focal lengths. Matching the ratio output to 1.0 confirms equivalent fields of view.