APS-C to Full Frame Focal Equivalence

Focal length equivalence: APS-C vs full-frame

A lens always projects its true focal length. The catch is that a smaller APS-C sensor only grabs a tighter slice of that image circle, so the angle of view comes out narrower. To line up framing across formats, multiply the APS-C focal length by the crop factor: f_equiv = f_real × crop. Canon APS-C runs a 1.6 crop; Nikon, Sony, Fujifilm and Pentax APS-C bodies sit at 1.5. Mount a 50 mm lens on a 1.5× body and it frames like a 75 mm on full-frame.

Aperture carries an equivalent value too, once you start comparing depth of field and total light gathered. Multiply the f-number by that same crop factor: an f/1.4 lens on APS-C gives roughly the depth of field and noise of an f/2.1 on full-frame at the matched framing. The ISO 517 standard formalizes f-number nomenclature, and you get the angle of view from 2 · arctan(sensor/(2f)).

Applications

Picking a portrait lens, where the 85 mm full-frame look actually comes from a 56 mm on APS-C. Building a landscape kit when you genuinely need a wide angle, since a 24 mm turns into a normal 36 mm on crop. Reading used-lens listings without getting fooled. And keeping creative choices consistent across a hybrid kit that mixes full-frame and crop bodies.

FAQ

Does the lens physically zoom in on APS-C? No. The optics don't change at all. The sensor just crops the projected image, which looks like zooming once you compare framing at the same print size.

Should I apply the crop factor to aperture as well? Depends what you're after. For exposure (shutter and ISO settings) no, since f/2.8 is f/2.8 on any sensor. For depth of field and total noise at the same framing, yes, multiply by the crop.

What crop factor do Micro Four Thirds and medium format use? MFT sits at 2.0×, which makes a 25 mm behave like a 50 mm. Fujifilm GFX medium format is about 0.79×, so a 63 mm lands at a 50 mm equivalent.