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The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror.. The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power.
Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f-number, written f /N, where N is the f-number given by the ratio of the focal length f to the diameter of the entrance pupil D: =. This ratio is related to the image-space numerical aperture when the lens is focused at infinity. [3]
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 1.26× – 1.29× for Canon (1.35× for Sigma "H") APS-H format, 1.5× for Nikon APS-C ("DX") format (also used by Sony, Pentax, Fuji, Samsung and others), 1.6× for Canon APS-C format, 2× for Micro Four Thirds format, 2.7× for 1-inch ...
A single-element camera lens is as long as its focal length; for example, 500 mm-focal-length lens requires 500 mm from the lens to the image plane. A telephoto lens is made physically shorter than its nominal focal length by pairing a front positive imaging cell with a rear magnifying negative cell.
The 1.5 indicates that the angle of view of a lens on the camera is the same as that of a 1.5 times longer focal length on a 35 mm full-frame camera, which explains why the crop factor is also known as a focal-length multiplier. For example, a 28 mm lens on the DSLR (given a crop factor of 1.5) would produce the angle of view of a 42 mm lens on ...
A normal lens typically has an angle of view that is close to one radian (~57.296˚) of the optical system's image circle. [citation needed] For 135 format (24 x 36 mm), with an escribed image circle diameter equal to the diagonal of the frame (43.266 mm), the focal length that has an angle of one radian of the inscribed circle is 39.6 mm; the focal length that has an angle of one radian of ...
The effective focal length is nearly equal to the stated focal length of the lens (F), except in macro photography where the lens-to-object distance is comparable to the focal length. In this case, the absolute transverse magnification factor ( m ) ( m = S 2 / S 1 {\displaystyle m=S_{2}/S_{1}} ) must be taken into account:
As with other types of camera lenses, the focal length is usually expressed in a millimeter value written on the lens, for example: a 500 mm lens. The most common type of long-focus lens is the telephoto lens , which incorporate a special lens group known as a telephoto group to make the physical length of the lens shorter than the focal length.