Diaphragm

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About Diaphragm

Diaphragm

1 optical
The diaphragm is variable narrowing through a thin sheet that is located between the objective lens, allows graduate the amount of light entering the camera. It is usually a disk or set of fins provided on the lens of a camera in a way that restricts the passage of light, usually tightly. The progressive opening of the diaphragm changes are specified using the f-number, which is the ratio between focal length and effective aperture diameter.
Evolution

The diaphragm is the part of the camera that determines the size of the opening. In its simplest form, used in older cameras, it was only a perforated plate.

The original adjustable diaphragms consisted of a metal plate with several holes of different sizes that slid or rotated to the target. Nearly all adjustable diaphragms adopt the design called the iris, which consists of a set of overlapping metal flakes that determine each aperture diameter, roughly circular and continuously variable that is controlled through a ring provided in the lens body.

The diaphragm can be mounted in front or behind it, although in composite designs is almost always in the center of the optical system. The so-called diaphragm-shutter is a similar mechanism in which the lamellae can fully close the opening, thus combining in one device the functions of the diaphragm and the shutter. This provision prevents the reading course and through the lens frame.
[Edit] Diaphragm and number f
Main article: number f (optical)

The ability of a target to let the light is called light. One way to indicate this luminosity by the number f is defined as the division of the lens focal length by the effective aperture diameter. This relationship gives rise to a standard scale in progression of \ sqrt (2): 1 1.4 2 2.8 4 5.6 8 11 16 22 32 45 etc. The jump from a value called the next step. The minimum value that may have the f number is 0.3. Although this value is unachievable in practice.

Strictly speaking, the number f does not depend on lens focal length, but the distance that is the optical center (the single node of a thin lens with the same power as the objective) of the image formed. When the lens is focused "at infinity" this distance proves to be the inverse of the power of the lens (focal length). So the focus at other distances the light changes, decline. This variation is minimized so that it is within the third step of the theoretical luminosity contributed by the number f. In movies it takes into account this variation and use two numbers to indicate the luminosity: the f and t The f is calculated and used to determine the depth of field. The f t is the number theory for real luminosity measurement. These numbers do not usually differ by more than one third of way.

Vlad

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