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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]
where f is the focal length of the lens. [1] Note that this measurement is valid only when the beam size is measured at the rear focal plane of the lens, i.e. where the focus would lie for a truly collimated beam, and not at the actual focus of the beam, which would occur behind the rear focal plane for a divergent beam.
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 ...
The effect is especially noticeable the closer the camera is to the subject, as its amount increases the shorter the focal length is at the same field size. One notable director that frequently employs rectilinear ultra wide angle lenses in order to achieve a distinctive signature style defined by extreme perspective distortion is Terry Gilliam.
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:
The term telephoto ratio refers to the physical length of a lens divided by its focal length; where long-focus lenses have a telephoto ratio around 1, telephoto lenses have a ratio less than 1. As an example, one modern lens ( Canon EF 400 mm f /4 DO IS ) achieves a telephoto ratio of 0.58 in part due to a front (converging) lens group which ...
A zoom lens is a system of camera lens elements for which the focal length (and thus angle of view) can be varied, as opposed to a fixed-focal-length (FFL) lens . A true zoom lens or optical zoom lens is a type of parfocal lens , one that maintains focus when its focal length changes. [ 1 ]