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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.
The camera equation, or G#, is the ratio of the radiance reaching the camera sensor to the irradiance on the focal plane of the camera lens. [8] The maximum usable aperture of a lens is specified as the focal ratio or f-number, defined as the lens's focal length divided by the effective aperture (or entrance pupil), a dimensionless number. The ...
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. Two lens-sensor combinations with the same 35 mm equivalent focal length are expected to have the same angle of view.
A 100 mm focal length f /4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length f /2 lens has an entrance pupil diameter of 50 mm. Since the area is proportional to the square of the pupil diameter, [6] the amount of light admitted by the f /2 lens is four times that of the f /4 lens.
Zoom lenses - variable focal length lenses. Zoom lenses cover a range of focal lengths by utilising movable elements within the barrel of the lens assembly. In early varifocal lens lenses, the focus also shifted as the lens focal length was changed. Varifocal lenses are also used in many modern autofocus cameras as the lenses are cheaper and ...
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 ...
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. [4]
There is no precise definition of the term, but lenses marketed as "standard zoom" usually cover a range of at least 30mm to 70mm in terms of 35mm equivalent focal length with an optical zoom ratio of 2.5× (e.g. 28-70mm) to 5× (e.g. 24-120mm) — the most common being 3× (e.g. 24-70mm). [1]