Crop factor

In digital photography, the crop factor of an image sensor format is related to the ratio of the dimensions of a camera's imaging area compared to a reference format; most often, this term is applied to digital cameras, relative to 35 mm film format as a reference. In the case of digital cameras, the imaging device would be a digital sensor. The most commonly used definition of crop factor is the ratio of a 35 mm frame's diagonal (43.3 mm) to the diagonal of the image sensor in question; that is, CF=diag_35mm / diag_sensor. Given the same 3:2 aspect ratio as 35mm's 36 mm × 24 mm area, this is equivalent to the ratio of heights or ratio of widths; the ratio of sensor areas is the square of the crop factor.

The crop factor is used to compare the field of view and image quality of different cameras with the same lens: The crop factor is sometimes referred to as the focal length multiplier ("FLM") since multiplying a lens focal length by the crop factor or FLM gives the focal length of a lens that would yield the same field of view if used on the reference format. For example, a lens with a 50 mm focal length on an imaging area with a crop factor of 1.6 with respect to the reference format (usually 35 mm) will yield the same field of view that a lens with an 80 mm focal length will yield on the reference format. If it is desired to capture an image with the same field of view and image quality but different cameras, the aperture and ISO settings also need to be adjusted with respect to the crop factor. It is important to note that the focal length of the lens does not change by using a smaller imaging area; the field of view is correspondingly smaller because a smaller area of the image circle cast by the lens is used by the smaller imaging area.

The term format factor is sometimes also used, and is a more neutral term that corresponds to the German word for this concept, Formatfaktor.

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 a DSLR "acts like" its focal length has been multiplied by 1.5, which means that it has the same field of view as a 75 mm lens on the film camera that they are 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.

...
Wikipedia