In photography, the 35 mm equivalent focal length is a measure that indicates the angle of view of a particular combination of a camera lens and film or sensor size. The term is useful because most photographers experienced with interchangeable lenses are most familiar with the 35 mm film format.
On any 35 mm film camera, a 28 mm lens is a wide-angle lens, and a 200 mm lens is a long-focus lens. However, now that digital cameras have mostly replaced 35 mm cameras, there is no uniform relation between the focal length of a lens and the angle of view, since the size of the camera sensor also determines angle of view, and sensor size is not standardized as film size was. The 35 mm equivalent focal length of a particular lens–sensor combination is the focal length that one would need for a 35 mm film camera to obtain the same angle of view.
Most commonly, the 35 mm equivalent focal length is based on equal diagonal angle of view. Alternatively, it may sometimes be based on horizontal angle of view. Since 35 mm film is normally used for images with an aspect ratio (width-to-height ratio) of 3:2, while many digital cameras have a 4:3 aspect ratio, which have different diagonal-to-width ratios, these two definitions are often not equivalent.
35 mm equivalent focal lengths are calculated by multiplying the actual focal length of the lens by the crop factor of the sensor. Typical crop factors are 1.5x for Nikon APS-C format, 1.6x for Canon APS-C format, 2x for Micro Four Thirds format, and crop factors of 5x to 6x for compact digital cameras. Quoted 35 mm equivalent focal lengths typically ignore depth of field, which is deeper for the shorter focal length lenses on smaller sensor cameras, however equivalent depth of field can be calculated the same way using the crop factor. For example, a 50mm f/2 lens on a 2x crop factor Micro Four Thirds camera would be equivalent to a 100mm f/4 lens on a Full-frame digital SLR in terms of field of view, depth of field, diffraction effects, and the total amount of light gathered from the scene.
Read more about 35 Mm Equivalent Focal Length: Conversions
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