F-number

Aperture range is a term used in photography and optics. It refers to the totality of all possible aperture settings of a lens, sorted from largest to smallest aperture.

F-number

In the f-numbers are relative values ​​obtained by dividing the focal length f by the diameter of the entrance pupil. The full spelling of the geometrical aperture value ( BZ) was accordingly f / (BZ ); "F / 4" corresponds to the specified camera lenses with f-number 4

The value at full aperture is referred to as light intensity. A lens with Aperture 2 has at 50 mm focal length, an effective opening cross- section (diameter of the entrance pupil ) of 25 mm, the entrance pupil of a 135 mm telephoto lens the same light intensity, however, already has a diameter of 67.5 mm.

The f-number is the denominator of a fraction; This explains the apparent contradiction that a larger f-number indicates a smaller aperture.

Today's dazzling array of

Whole stop increments

The aperture range is designed so that the falling amount of light through the lens from f-stop to f-stop

The aperture diameter D increases or decreases from f-stop to f-stop by a factor of √ 2, 1 / √ 2, which doubled or halved surface and amount of light. This step corresponds to the usual exposure time series and enables easy adjustment of aperture and shutter speed for a given illumination.

Each f-number k is calculated from the above by multiplying by √ 2. Accordingly, any aperture setting can be represented by the formula k = ( √ 2 ) (n -1) ( n ∈ N, n0 = 1, n1 = n0 1 ) can be calculated. However, the mathematically exact values ​​do not coincide exactly with the usual aperture rows Convention, but are rounded, thus resulting in the following series:

Half stops

(rounded)

Half aperture settings can be adjusted in many manual lenses on the aperture ring. In general, they are not labeled separately.

Third - stop increments

(rounded)

Modern cameras are often able to hire third - stop increments. This is usually done electronically via a dial on the camera or the automatic exposure control of the camera. Third stage or other, different values ​​are often given in order to describe the initial opening ( light intensity ) of lenses, if it does not match with a whole f-stop. Many standard zoom lenses for example have a top opening from 3.5 to 5.6, depending on the focal length.

Old dazzling array

The pre-World War usual, so-called old dazzling array used the f-number of 3.2 ( square root of 10) as a base and had the following gradation:

A special feature of the dazzling array of Minolta - Flex ( 1936), a twin-lens reflex camera for roll film of type 120 and the recording format 60 mm × 60 mm, on: On it is a mixture of "old" and "new" aperture series is engraved:

Many older cameras and lenses do not have a click-stop diaphragm, as it is in lenses for enlargers usual to switch off the stop aperture, so that any intermediate values ​​can be set.

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