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本帖最后由 煮鱼大嘬 于 2011-9-1 17:31 编辑
回复 a_l_n 的帖子
就算不幸知道了又有什么用？ 兄台是上光所的吗？
镜头生产是物理 化学（镜片化学合成） 数学 计算机 人体工程 的综合我们需要全知道吗？ 如果你认为需要知道请分享一下。谢谢！
在光学中，一个光学系统中的焦比(英文f-number，或称"F比例"、"相对孔径"、"光圈值"等)表达了镜头的焦距和光圈直径大小的关系。简单来说，焦比等如焦距数除以孔径数。焦比是无因次量的，它代表了摄影学中的一个重要概念：镜速(Lens speed)的量。
The f-number (f/#) is often notated as N and is given by
where f is the focal length, and D is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16. The greater the f-number, the less light per unit area reaches the image plane of the system; the amount of light transmitted to the film (or sensor) decreases with the f-number squared. Doubling the f-number increases the necessary exposure time by a factor of four.
The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 200 mm focal length lens with a setting of f/4 will have a pupil diameter of 50 mm. The 200 mm lens's f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
In other types of optical system, such as telescopes and binoculars, the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).
T-stops
Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The T in T-stop stands for transmission.[5] The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness. Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly. Modern cinematographic lenses now usually tend to be factory-calibrated in T-stops. T-stops measure the amount of light transmitted through the lens in practice, and are equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number. Consequently, the depth of field for a given T-number will be slightly less than that when the f-number is set to the same value, since the aperture diameter is slightly greater. In recent years, advances in lens technology and in film and sensor exposure latitude have reduced the need for T-stop values, although T-stops are still considered industry standard for cinematographic lenses
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发表于 2011-8-26 21:14
楼主
如我所料的 过万了 这2镜头 
