Diaphragm (optics)

The aperture is a (usually mechanical ) device of cameras, with the aid of the light transmission through the optical system ( lens) may be changed. It is usually a multi-plate (also called iris ) is executed, in which circularly arranged lamella plates move into each other so that the light passage narrower or wider and so the incident light beam is smaller or larger. The aperture is positioned in the beam path such that it acts only as an aperture stop, and not as a field stop. From the choice of the f-number depend primarily exposure time and depth of field. The f-number is the ratio of focal length to aperture width.

Influence on exposure time

The aperture directly affects the strength of the exposure of the image sensor, film or photo paper from: The larger the aperture value, the smaller the aperture and the less light passes through the lens. The lens is in the f-number setting 5.6 four times less light than at 2.8. Thus, the exposure of the film or recording sensor is used in conjunction with the exposure time adjusted (the ratio is described in light value ). General:

  • The larger the f-number, the longer the exposure time (for the smaller the aperture).
  • The smaller the F number, the shorter the exposure time ( because the larger the aperture).

An automatic exposure, in which the exposure time is manually set, and the aperture is controlled by the camera in dependence on the available quantity of light is referred to as auto iris. Alternatively, in the automatic mode, the aperture is pre-selected and controlled by the exposure time of the camera. If both exposure time and aperture according to a programmed pattern of the camera controlled, one speaks of an automatic program. Some cameras allow for the change in the set by the program automatic aperture and shutter speed combination using manual controls.

Impact on depth of field

On the other hand, the aperture affects the depth of field: The larger the aperture value, and thus a smaller aperture not only the effective amount of light is reduced, the blur circles are smaller by her light cone. Consequently, increasing the area of ​​the subject, which is still accepted as a sharp, to the permissible limit ( circle of confusion diameter, here 0.1 mm ) is reached. The area of ​​sharp imaging ( depth of field) so increases when closing the aperture. It follows:

  • The larger the f-number is, the wider the depth of field ( because the smaller the aperture).
  • The smaller the f-number, the narrower the depth of field ( because the larger the aperture).

In the language of photography, the term aperture is often used as shorthand for lens aperture, and for example, instead of talking about large aperture of large aperture. This usage is common, but can lead to misunderstandings, since a large aperture of a small f-number (and vice versa) corresponds.

Some single lens reflex cameras, the photographer can use the preview button to control the depth of field. The camera then activates the working aperture.

F-number and aperture range

Aperture is expressed as F-number. It results from the ratio of focal length to aperture size of the optics ( lens). The aperture of the lens is the reciprocal of the smallest f-number, which is the largest relative aperture. This is often expressed as a fraction of the focal length f, such as f / 2, (English f-stop). Larger f-numbers mean at the same focal length accordingly stronger dimming, so a lower incidence of light. The f-number is set at mechanical cameras on the lens ring in modern electronic cameras also have controls on the camera body (body ).

With lenses that are specifically designed for videography, the aperture is specified as a so-called T-Stop (English Transmission Stop) whose values ​​are actually measured transmission values ​​, rather than relatively imprecise, only calculated to be considered a fraction of the focal length.

As dazzling array is a series of f-stops, each involved twice the amount of light. The adjacent values ​​of a dazzling array of stand, as they relate to the diameter of the opening, always in proportion, so that the open area varies by square of these values. Thus, the adjustment of the aperture compensate for a correspondingly opposite adjustment of shutter speed to value by a value.

Influence on the sharpness

Usually lenses make more from the sharper, the more the aperture is closed. From a certain aperture, diffraction of light rays at the aperture becomes noticeable and small diffraction disk are noticeable, the focus then decreases again.