APS-C today refers to sensor sizes approximately equaling 24×16 mm, leading to a crop factor of between 1,5 and 1,7 (compared to full-frame).

Depending on the marque of APS-C camera, the actual dimension (and thus crop factor) of the sensor vary somewhat, ranging from 1,52 (e.g. Nikon, Pentax, Fujifilm) through 1,6 (Canon), to 1,7 (Sigma)

Originally, APS was a novel (but since discontinued) film format (for film cameras), called Advanced Photo System. APS film was produced in three variants:
– APS-H (for high definition), 30.2 × 16.7 mm; aspect ratio 16:9, equalling approximately a 1,25 crop factor,
– APS-C (for Classic), 25.1 × 16.7 mm; aspect ratio 3:2, equalling approximately a 1,44 crop factor, and
– APS-P (for Panoramix), 30.2 × 9.5 mm; aspect ratio 3:1.

After the advent of digital SLR cameras featuring sub-full-frame sensors, the APS-designations were revitalised (even though the sensor sizes and sizes of original APS film did not entirely match), with most 1,5–1,7 crop factor sensors being given the APS-C designation and some (most notably Canon’s original 1D-series with a 1,3 crop factor) being given the APS-H designation.

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