Introduction
The terms crop factor and focal length multiplier were coined in recent years in an attempt to help 35 mm film format SLR photographers understand how their existing ranges of lenses would perform on newly introduced DSLR cameras which had sensors smaller than the 35 mm film format, but often utilized existing 35 mm film format SLR lens mounts. Using an FLM of 1.5, for example, a photographer might say that a 50 mm lens on his DSLR "acts like" its focal length has been multiplied by 1.5, by which he means that it has the same field of view as a 75 mm lens on the film camera that he is more familiar with. Of course, the actual focal length of a photographic lens is fixed by its optical construction, and does not change with the format of the sensor that is put behind it.
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.
Further information: Image sensor format#Table_of_sensor_sizesRead more about this topic: Crop Factor
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