Crop Factor & Equivalent Focal Length Calculator
What your lens 'really is' on any sensor: equivalent focal length, equivalent aperture for DoF, and the light-gathering truth behind the equivalence wars.
The part spec sheets omit: crop factor applies to the f-number too — for depth of field and TOTAL light collected. A 25mm f/1.4 on Micro 4/3 frames and blurs like a 50mm f/2.8 on full frame. Per-area exposure (metering) is unchanged; that's why both claims in the equivalence wars are 'right.'
Formula
⚠️ Optical estimates based on standard formulas and circle-of-confusion conventions — lens markings, sensor specs and real-world testing have the final word.
What your lens 'really is' on any sensor: equivalent focal length, equivalent aperture for DoF, and the light-gathering truth behind the equivalence wars.
About Crop Factor & Equivalent Focal Length Calculator
Mount the same 35mm lens on three different cameras and you've taken three different photographs: full-frame sees a moderate wide, APS-C a normal, Micro 4/3 a short tele. Crop factor — the ratio of sensor diagonals — is the exchange rate between these worlds, and it converts more than focal length: depth of field and total light gathered follow it too. This calculator gives the full-frame-equivalent focal length, the equivalent aperture nobody prints on the lens, and the actual field of view in degrees.
How to use Crop Factor & Equivalent Focal Length Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula equivalent FL = f × crop; equivalent aperture (DoF/total light) = N × crop; crop = 43.27 mm ÷ sensor diagonal substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Crop Factor & Equivalent Focal Length Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula equivalent FL = f × crop; equivalent aperture (DoF/total light) = N × crop; crop = 43.27 mm ÷ sensor diagonal with sources cited on the page
- ✓The part spec sheets omit: crop factor applies to the f-number too — for depth of field and TOTAL light collected. A 25mm f/1.4 on Micro 4/3 frames and blurs like a 50mm f/2.8 on full frame. Per-area exposure (metering) is unchanged; that's why both claims in the equivalence wars are 'right.'
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Does a crop sensor 'turn my 50mm into a 75mm'?+
It changes nothing inside the lens — focal length is physics, not framing. What changes is how much of the image circle the sensor keeps: APS-C crops the centre, producing the FIELD OF VIEW a 75mm would give on full frame. Perspective, being a function of where you stand, is untouched. The honest sentence: 'a 50mm on APS-C frames like a 75mm.' For most practical decisions (what lens do I need for this shot?) the equivalent number is exactly the right currency.
Why do you multiply the f-number by the crop factor too?+
Because depth of field and background blur depend on the physical aperture diameter relative to the (equivalent) framing: a 50mm f/1.8 on APS-C frames like 75mm but blurs like f/2.7 on full frame — same shot, visibly deeper focus. Total light captured (sensor area × per-area intensity) scales the same way, which sets the noise floor at equal final-image size. What does NOT change is per-area exposure: f/1.8 meters as f/1.8 on any sensor — both sides of every internet equivalence war are defending different, correct statements.
What are the standard crop factors?+
Against the 43.27mm full-frame diagonal: APS-C 1.5 (Nikon/Sony/Fuji) or 1.6 (Canon); Micro 4/3 2.0; 1-inch 2.7; typical flagship-phone main sensors 3.5–4.5; going the other way, medium format 44×33 is 0.79 (a 'reverse crop' — an 80mm f/2.8 frames like 63mm f/2.2). The number is just diagonal ratio; this tool's table carries the sensor dimensions underneath it.
Should I buy lenses by real or equivalent focal length?+
Think in equivalents, buy in reals: decide the framing language first (24-eq wide, 50-eq normal, 85-eq portrait — these define genres regardless of system), then divide by your crop factor to shop. The classic trio on APS-C: 16, 33 and 56mm. Mind the aperture column when comparing systems: a 56mm f/1.2 APS-C lens and an 85mm f/1.8 full-frame lens are nearly the same instrument (85-eq, f/1.8-eq) — which reframes many price comparisons honestly.
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