How it works · 08
Format, crop factor and equivalence
Why sensor size changes what a lens sees, what 'full frame' and 'APS-C' really mean, and how to compare lenses across formats.
Try it: the same picture on two formats
All formats to scale- B's crop factor relative to A
- B's area relative to A
- Same view on B
- Same depth of field on B
Where the names come from
35mm and full frame. Cinema film is 35 mm wide. When Oskar Barnack built the Leica, he laid the film sideways and doubled the cine frame to 24 × 36 mm. Kodak’s 135 cartridge made that frame the world standard. Digital sensors of the same size were called full frame because most early sensors were smaller.
APS-C. The Advanced Photo System film of 1996 had a “classic” C print format of 25.1 × 16.7 mm. Early digital SLR sensors were about that size, so they were called APS-C, although the film itself was soon forgotten.
Inch types. Small sensors are named “1-inch” or “1/2.3-inch”, sizes inherited from the glass tubes of 1950s video cameras. A “1-inch” sensor is only about 13.2 × 8.8 mm.
Crop factor
A lens projects a round image; the sensor takes a rectangle from the middle. A smaller sensor takes a smaller piece, so the same lens shows a narrower view. The crop factor is the full-frame diagonal (43.27 mm) divided by the sensor’s diagonal: about 1.5 for most APS-C sensors, 1.6 for Canon’s, 2 for Four Thirds and 2.7 for 1-inch. A 50 mm lens on APS-C frames like a 75–80 mm lens on full frame.
Equivalence
To get the same photograph on two formats (same framing, same depth of field, same total light) multiply both the focal length and the f-number by the crop factor. A 35 mm f/1.4 lens on APS-C behaves like a 52 mm f/2.1 lens on full frame. The exposure setting is still f/1.4: brightness per area is the same, but the smaller sensor collects less light in total, which is why it shows more noise.