A 35mm lens does not see the same picture on every camera — the sensor’s size crops the view, and the conversion factor has its own name. This Crop Factor Calculator takes your sensor format and a lens’s true focal length and returns the full-frame equivalent focal length: the number that tells you how the combination actually frames, in the common language every photographer shares.
Worked Example: A 35mm f/1.8 on APS-C
Nikon/Sony APS-C carries a 1.5× factor, so the 35mm lens frames like a 35 × 1.5 = 52.5mm lens on full frame — the classic “nifty fifty” view, which is why 35mm is the normal-lens choice on crop bodies while 50mm is normal on full frame. The aperture equivalent for depth of field scales by the same factor: f/1.8 × 1.5 = f/2.7 — from the same position, the crop camera at f/1.8 produces the depth of field a full-frame camera would at f/2.7 (its light gathering per unit area is unchanged; only the depth of field equivalence moves, a distinction the next section insists on). Where the factor comes from is pure geometry: crop factor is the ratio of sensor diagonals, full frame’s diagonal being √(36² + 24²) = 43.3 mm. An APS-C sensor at 23.5 × 15.6 mm has a 28.2 mm diagonal — 43.3 / 28.2 = 1.53, marketed as 1.5. Canon’s slightly smaller APS-C (22.3 × 14.9, diagonal 26.8 mm) gives 1.61, marketed 1.6; Micro Four Thirds (17.3 × 13, diagonal 21.6) gives exactly 2.0; the 1-inch type (13.2 × 8.8, diagonal 15.9) gives 2.7. Because the factor is a diagonal ratio, it converts not just focal length but the whole framing — the angle of view a lens delivers on any format matches the angle of the equivalent focal length on full frame, which is the entire point of keeping one reference format as the lingua franca.
What Equivalence Does and Does Not Say
Equivalence is a statement about the picture, not about the exposure or the glass, and three boundaries keep it honest. Framing and depth of field: fully equivalent — a 35mm f/1.8 on 1.5× crop, shot from the same spot, frames and blurs like a 52.5mm f/2.7 on full frame (with matching shutter and appropriately scaled ISO, total light and noise join the equivalence too, which is the deeper truth behind “crop cameras are worse in low light”: the sensor collects less total light at the same settings). Exposure per unit area: unchanged — f/1.8 is f/1.8 for exposure purposes on any format, and a light meter does not care about your crop factor. The lens itself: unchanged — focal length is a physical property of the optics; a 35mm lens is a 35mm lens on every body, and “the crop factor multiplies your focal length” is a shorthand for the framing equivalence, not a change in the glass. Medium format inverts the direction: on a 44 × 33 mm sensor (0.8×), a 50mm lens frames like a 40mm on full frame — a “reverse crop” — and the same calculator handles it by dividing where it usually multiplies. Master these boundaries and the format wars become a spec-sheet translation exercise: every system’s lenses can be compared in one currency, and the comparison this page automates is the first step of any cross-system decision.
Choosing Lenses Across Formats
The calculator earns its keep at purchase time, where the same desired view wears different numbers per system. Want the classic 85mm portrait view? On full frame it is an 85mm lens; on 1.5× crop it is a 56mm (85 / 1.5 ≈ 57 — the 56mm f/1.4 lenses that exist for exactly this reason); on Micro Four Thirds it is a 42.5mm. Want a 24mm wide view for interiors? Crop shooters reach for 16mm, MFT shooters for 12mm. The telephoto direction is where small formats press their advantage: a 300mm lens on MFT frames like 600mm on full frame — reach that wildlife and sports photographers buy deliberately, paid for in the depth-of-field and total-light terms the equivalence describes. Phones live at the far end of the same table (main cameras around 24–26mm equivalent from tiny sensors and ~5mm physical lenses), which is why their spec sheets quote equivalents in the first place. And video shooters meet the factor twice: many cameras crop further in video modes, stacking a second factor on the format’s own — a “1.5× crop mode” on a 1.5× body behaves as 2.25×, and the only safe procedure is to multiply the factors and re-run the equivalence before the shoot, not during it.
Angle of View: The Geometry Behind the Equivalence
The equivalent focal length is a proxy; the physical quantity it preserves is the angle of view — the width of the world the frame admits, computed from the sensor dimension and the focal length by 2 × atan(d / 2f). Put numbers to the worked example: the 35mm lens on full frame gathers a horizontal angle of 2 × atan(18/35) = 54.4° (63.4° on the diagonal). Move that lens to the 1.5× body and the sensor only keeps the centre of the image: the horizontal angle narrows to what a 52.5mm lens sees on full frame — 2 × atan(18/52.5) = 37.8°. Nothing about the lens changed; the crop is a window cut from the same projected image, which is literally why the factor is called a crop and why post-cropping a full-frame file to the APS-C area produces the identical photograph (at lower resolution). Angle of view is the dialect worth thinking in when equivalence talk gets heated: two systems are “equivalent” for a shot when angle of view, depth of field and total light match, and the first of the three is pure geometry of the kind this paragraph computes. It also runs the other way — given a required angle (a room’s width from a fixed doorway, a stage from a fixed seat), the formula names the focal length per format directly, and the calculator’s equivalent is then the shopping translation.
Converters, Speed Boosters and Stacked Factors
Two accessories rewrite the equivalence and deserve a place in the mental table. A teleconverter multiplies the lens’s focal length and its f-number by its factor: a 300mm f/2.8 behind a 1.4× converter behaves as a 420mm f/4 — on any format — and the crop factor then applies on top if the body is a crop body (420 × 1.5 = 630mm equivalent framing, with f/4’s exposure and f/6-equivalent depth of field on full-frame terms). A focal reducer (“speed booster”) runs the film backwards: a 0.71× reducer on a full-frame 50mm f/1.4 mounted to a crop body yields an effective 35.5mm and gathers a stop, landing near the original’s full-frame behaviour — the adapter exists precisely to refund the crop. The discipline that survives all combinations: factors multiply. Format factor, converter factor, video-crop factor and reducer factor compose into one product, and the calculator’s single multiplication is applied once per factor in the chain. Write the chain down before a shoot that stacks two of them; the arithmetic is trivial and the 2 a.m. multiplication errors are not.
Common Mistakes
Believing the crop factor changes the lens (it changes the framing the sensor takes from the lens’s image circle). Applying the factor to exposure: f/1.8 exposes identically on every format. Forgetting the depth-of-field equivalence when comparing “fast” lenses across systems — an f/1.8 on MFT blurs like f/3.6 on full frame. Mixing up Canon’s 1.6 and everyone else’s 1.5 APS-C factors. And quoting a phone’s or compact’s physical focal length as if it were the equivalent — always convert before comparing views.
Related Tools
Continue with the Depth of Field Calculator, the PPI Calculator, the Note Frequency Calculator, or browse the full Other Calculators hub.
Frequently Asked Questions
What is crop factor?
The ratio of full frame’s sensor diagonal (43.3 mm) to your sensor’s diagonal. APS-C is 1.5× (Canon 1.6×), Micro Four Thirds 2.0×, 1-inch 2.7×. Multiply a lens’s focal length by it to get the full-frame equivalent view.
What is the full-frame equivalent of a 35mm lens on APS-C?
35 × 1.5 = 52.5mm — the standard “normal” view. That is why 35mm lenses are the normal-lens choice on crop bodies and 50mm lenses are normal on full frame.
Does crop factor change my aperture?
Not for exposure: f/1.8 meters the same on any format. For depth of field equivalence it scales: f/1.8 on a 1.5× body gives the depth of field of f/2.7 on full frame from the same position.
Why do crop cameras give more “reach”?
The smaller sensor records the centre of the image — the same framing a longer lens would give on full frame. A 300mm on Micro Four Thirds frames like 600mm; the trade is total light and depth of field, per the equivalence.
Is a 50mm lens still a 50mm lens on a crop body?
Physically, yes — focal length belongs to the lens. It frames like a 75mm (1.5×) on full frame, which is the only sense in which the “multiplied focal length” shorthand is true.
What about medium format?
The factor inverts: 44 × 33 mm digital medium format is about 0.8×, so a 50mm lens frames like a 40mm on full frame — wider, with shallower equivalent depth of field at the same f-number.