Crop factor

The crop factor is sometimes used to compare the field of view and image quality of different cameras with the same lens.

The terms crop factor and FLM (Focal Length Multiplier) were coined 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.

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.

Due to the statistics of photon shot noise, the desirable properties of signal-to-noise ratio (SNR) and sensor unity gain both scale with the square root of pixel area.

The larger sensor has the smaller crop factor and the higher signal-to-noise ratio.

Smaller, non-DSLR, consumer cameras, typically referred to as point-and-shoot cameras, can also be characterized as having a crop factor or FLM relative to 35 mm format, even though they do not use interchangeable lenses or lenses designed for a different format.

Reviewers also sometimes use the 35 mm-equivalent focal length as a way to characterize the field of view of a range of cameras in common terms.

[3] For example, the Canon Powershot SD600 lens is labeled with its actual focal length range of 5.8–17.4 mm.

A camera with a smaller sensor can be preferable to using a teleconverter, because the latter affects the f-number of the lens, and can therefore degrade the performance of the autofocus.

Shooting from the same position, with the same lens and same f-number as a non-cropped (full-frame) 35 mm camera, but enlarging the image to a given reference size, will yield a reduced depth of field.

As a result, the focal length that can be reliably hand-held at a given shutter speed for a sharp image is reduced by the crop factor.

The old rule of thumb that shutter speed should be at least equal to focal length (in millimetres) for hand-holding will work equivalently if the actual focal length is multiplied by the FLM first before applying the rule.

Many photographic lenses produce a more superior image in the center of the frame than around the edges.

When using a lens designed to expose a 35 mm film frame with a smaller-format sensor, only the central "sweet spot" of the image is used; a lens that is unacceptably soft or dark around the edges can give better results on a smaller sensor.

The outer, red box displays what a 24×36 mm sensor would see, the inner, blue box displays what a 15×23 mm sensor would see. (The actual image circle of most lenses designed for 35 mm SLR format would extend further beyond the red box than shown in the above image.)
A 50 mm (focal length) lens on an APS-C image sensor format (crop factor 1.6) images a slightly smaller field of view than a 70 mm lens on a 35 mm sensor format camera (full frame sensor). A 80 mm lens (1.6 × 50 mm = 80 mm) with a full frame camera gives the same field of view as this 50 mm lens and APS-C sensor format combination produces.
An APS-C format SLR (left) and a full-frame DSLR (right) show the difference in the size of the image sensors.
Some manufacturers provide both the real focal length and the 35 mm equivalent focal length
Field-of-view crop is cameras of different sensor size but the same lens focal length
Field-of-view crop in cameras of different sensor size but the same lens focal length.
Comparison of camera sensor formats