Back in the film days, the rectangle that captured the image on a standard SLR (the film) was one size: 24mm x 36mm (24mm x 36mm).
When the world switched over to digital camera, the film was replaced by image sensor. In the earlier years image sensors were significantly smaller than 35mm film.
In 2002, the first sensor that equaled the size of 35mm film was produced by Canon. That means Canon was the first mainstream camera manufacturer to produce a DSLR camera with a sensor the size of 35mm film.
From that time on wards, a full-frame digital SLR is a digital single-lens reflex camera (DSLR) fitted with an image sensor that is the same size as a 35 mm (36×24 mm) film frame.
Canon has three sensor sizes: full frame, 1.3x and 1.6x while Nikon has two different sensor sizes: full frame (FX) and 1.5x (DX). Most of the other manufacturers are in the same range, with Olympus being the notable exception, at 2x.
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