Most DSLRs on the market have nominally APS-C-sized image sensors, smaller than the standard 36 × 24 mm (35 mm) film frame. For example, many Canon DSLRs use an APS-C sensor that measures 22.2 mm × 14.8 mm. The result is that the image sensor captures image data from a smaller area than a 35 mm film SLR camera would, effectively cropping out the corners and sides that would be captured by the 36 mm × 24 mm 'full-size' film frame.

Because of this crop, the effective field of view (FOV) is reduced by a factor proportional to the ratio between the smaller sensor size and the 35 mm film format (reference) size.

For most DSLR cameras, this factor is 1.3–2.0×. For example, a 28 mm lens delivers a moderately wide-angle FOV on a 35 mm format full-frame camera, but on a camera with a 1.6 crop factor, an image made with the same lens will have the same field of view that a full-frame camera would make with a ~45 mm lens (28 × 1.6 = 44.8). This narrowing of the FOV is a disadvantage to photographers when a wide FOV is desired. Ultra-wide lens designs become merely wide; wide-angle lenses become 'normal'. However, the crop factor can be an advantage to photographers when a narrow FOV is desired. It allows photographers with long-focal-length lenses to fill the frame more easily when the subject is far away. A 300 mm lens on a camera with a 1.6 crop factor delivers images with the same FOV that a 35 mm film format camera would require a 480 mm long focus lens to capture.