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Nine-blade iris Pentacon 2.8/135 lens with 15-blade iris Aperture mechanism of Canon 50mm f/1.8 II lens, with five blades In the human eye, the iris (light brown) acts as the diaphragm and continuously constricts and dilates its aperture (the pupil) A 750nm titanium-sapphire laser beam passing through an iris diaphragm, while opening and closing the iris.
The f-number N is given by: = where f is the focal length, and D is the diameter of the entrance pupil (effective aperture).It is customary to write f-numbers preceded by "f /", which forms a mathematical expression of the entrance pupil's diameter in terms of f and N. [1]
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 development of the 'Tessar' lens is discussed by Nasse, [11] and shows its transformation from an f /6.3 lens for plate cameras using the original three-group configuration through to an f /2.8 5.2 mm four-element optic with eight extremely aspheric surfaces, economically manufacturable because of its small size. Its performance is 'better ...
However, this specification is insufficient to compare lenses for different cameras, because field of view also depends on sensor size. For example, a 50 mm lens mounted on a Nikon D3 (a full-frame camera) provides approximately the same field of view as a 32 mm lens mounted on a Sony α100 (an APS-C camera). Conversely, the same lens can ...
So a standard 50 mm lens for 35 mm photography acts like a 50 mm standard "film" lens even on a professional digital SLR, but would act closer to a 75 mm (1.5×50 mm Nikon) or 80 mm lens (1.6×50mm Canon) on many mid-market DSLRs, and the 40-degree angle of view of a standard 50 mm lens on a film camera is equivalent to a 28–35 mm lens on ...
In photography, the size of the entrance pupil (rather than the size of the physical aperture stop) is used to calibrate the opening and closing of the diaphragm aperture. The f-number (also called the ' relative aperture '), N, is defined by N = f / E N, where f is the focal length and E N is the diameter of the entrance pupil. [2]
An APS-C type lens often does not have the coverage to cast light on the entirety of a 35mm sensor, causing dark corners in the images. [ 2 ] One common misconception about lenses is that focal length determines the angle of view for a lens, when in reality it is a combination of focal length, lens coverage, and sensor size.