APS-C
A sensor format roughly 23.5 by 15.6 millimetres, giving a crop factor of about 1.5× against full frame.
Formula / rule
Crop factor ≈ 1.5 (1.6 Canon)
Measured in
mm
In depth
APS-C emerged as the practical compromise when full-frame sensors were prohibitively expensive, and it survived on merit: bodies and lenses are smaller and cheaper, and the extra reach flatters wildlife and sports work. Canon's variant is slightly smaller at 1.6×, which is why equivalence figures differ marginally between brands.
Examples in practice
A 300 mm lens on APS-C frames like a 450 mm on full frame, giving a wildlife photographer reach without the weight.