Crop factor

The outer, red box displays what a 24x36mm sensor would see, the inner, blue box displays what a 15x23mm sensor would display

Crop factor is the amount of photographic area lost in a DSLR camera compared to standard 35mm film. Most DSLRs on the market have image sensors smaller than 24x36mm, which is the standard size of a 35mm film frame. (For example, most of Canon's consumer-level DSLRs use a sensor that measures 15x23mm.) Lenses designed for 35mm cameras cast an image circle too large for the DSLR sensor. The result is that the image sensor captures image data only from the center of the circle, effectively cropping out the corners and sides that would be captured by the 24x36mm 'full-size' film frame.

Because of this crop, the effective field of view (FOV) is reduced by a factor proportional to the difference between the smaller sensor size and the full-frame that the lens was designed to work with.

For most digital cameras, this factor is 1.5 - 1.6X. For example, a 28mm lens delivers a moderately wide-angle FOV on a full-frame camera. However, on a camera with a 1.6X 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 ~45mm lens (28 X 1.6 = 44.8). This is a disadvantage to photographers when a wide FOV is desired -- ultra-wide perspectives become 'merely' wide; wide-angle perspectives 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 300mm lens on a camera with a 1.6X crop factor delivers images with the same FOV that a full-frame camera would require a 480mm lens to capture.

Some camera manufacturers have addressed the concerns of wide-angle lens users by designing lenses with shorter focal lengths. These lenses cast a smaller image circle that would not cover a 24x36mm frame, but is large enough to cover the smaller 15x23mm sensor in most DSLRs. Because they cast a smaller image circle, the lenses require less glass and are physically smaller and lighter than those designed for full-frame cameras. New lenses designed for specific use on DSLRs are subject to the same crop factor, depsite projecting a smaller image.

The standard focal length measurements are dependent on the camera sensor size, not the lenses. This causes confusion because a 18-70mm digital or regular lens will never be 18-70mm by conventional film camera standards. The entire range is multiplied by the crop factor when a digital camera is used. As cameras transition from film to digital so does the conventional understanding of the focal length numbers when used on the two different formats.

Magnification factor

The crop factor is sometimes referred to as "magnification factor." This usage commonly derives from the telephoto effect, in which lenses of a given focal length seem to produce greater magnification on crop-factor cameras than they do on full-frame cameras. It should be noted that the lens casts the same image no matter what camera it is attached to, and therefore produces the same magnification on all cameras. It is only because the image sensor is smaller in many DSLRs that a narrower FOV is achieved. The end result is that while the lens produces the same magnification it always did, the image produced on small-sensor DSLRs will be enlarged more to produce output (print or screen) that matches the output of a longer focal length lens on a full-frame camera.

External links

• DSLR Crop/Magnification Factor on The Luminous-Landscape