Aperture in photography explained

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Apertures are optic devices that limit the cross-section of radiation beams . Depending on the effect and design, covers are called different.

Aperture in photography explained
Iris diaphragm with 19 diaphragm blades in a large lens of a medium format camera

Designation accordance with the action

Depending on the effect, three pure forms of diaphragms can be distinguished from one another:


A pure aperture diaphragm is uniformly attached to the aperture of the optical device. Does not affect the size of the image. For this purpose, it should be considered that the same radiation flow is relative to the radiant power of the object.

In devices having a single image component (eg, a lens or a primary mirror), the aperture diaphragm is generally disposed close to it or is made by the lens or the edge of the mirror. In the eye, the iris acts as an opening stop. For more complex devices, such as camera lenses, the aperture diaphragm can also be in the object, in the image or between the image elements.

Aperture in photography explained
additional iris diaphragm

The images of the aperture diaphragm are called the entrance pupil in the object and the exit pupil in the image side.

In practice, almost any limitation of a beam through an aperture causes a beam passing through the cross section of the aperture at a different angle to be cut in a different ratio. This usually leads to a darkening of the image near the edge, which goes beyond the law Cos 4.  This phenomenon is called vignetting.

Aperture in photography explained
Iris diaphragm, functional principle


Field Blend

A pure field of view limits the image without affecting the brightness of the image.

It is located in the image plane (z. B. sensor chip of a digital camera ), the object plane (z. B. slide mounts in the slide projector) or in an intermediate image plane (z. B. in the microscope ). In practice, the field stop is usually not exactly at the object or image, so that the edge is never sharply defined.

The images of the visual field diaphragm are called the entrance hatch (on the object side) and the exit hatch (on the image side).

A field stop in an illumination beam path is also called field stop . It limits the illuminated area on the observed object. In a classic Köhler illumination with the microscope , it is usually designed as an adjustable iris diaphragm . In confocal technology , the field diaphragm is so small that the luminous field is only determined by the diffraction-related resolution limit of the microscope. In a Kreutzblende comes an aperture with crescent-shaped opening used.

Aperture in photography explained
Different apertures

Lens hood

A pure lens hood is located outside the beam path and affects neither the brightness of the image nor the size of the image section.

Aperture in photography explained
The aperture diaphragm is located on the image side between the tips of the dark gray components.


Photographic iris is often made as in an iris diaphragm. A lens mount acts as a fixed aperture diaphragm. But as it often reflects scattered light, a fixed opening ring is placed before or after.

When the magnification of negatives (photographs made on film or plate) is made of a metal plate, it is placed on a metal plate. Therefore, the paper is pressed to a large extent on the cassette (good for sharpness and without distortion), the image is considerably limited and created with a white margin not exposed on the paper. Of the larger glass negatives, contact copies are often made in a fixed-frame wooden cassette.

The body of the camera already generates its film guide, usually made of aluminum or plastic, which molds a rectangular boundary of the exposed surface of the film. The special characteristics of the contour (dimensions, rounding of corners, shadow of a filament) offer researchers clues about the type of camera used or even the copy of the camera. The frames of the slides form an optical frame around the film of the slide, which is somewhat smaller than the standard format (eg, small image 24 × 36 mm).

Aperture in photography explained
two-way adjustable slit diaphragm : mostly used as a field diaphragm (for example on the prism spectrometer )

Openings with a striking outline. Classically, the perspective of the keyhole with a circular opening and an attached trapezoid, which extends slightly downward, is the view not observed through a door. 4 squares at a small distance through a prison window with bars, many squares through a ventilation grid.

Small hatches, often yellow in the back of a movie. Stock Photography in the back of a movie. Some miniature cameras allow this to be read on the label of the film cartridge.

Slot diaphragms generally limit the path of the beam only in one direction as a field diaphragm. In spectrometers and related optical devices, they are called optical gaps, they usually have a width (order of magnitude of 1 mm when it comes to the visible spectral range) and their height (order of magnitude of 20 mm) adjustable.

They serve on the input side as secondary light sources in a well defined and usable way, on the output side, for example, of monochromators as selection agents for certain wavelengths (ranges) and rotate as secondary light sources. The width of the gap is often negligible.

In ophthalmology, a non-adjustable orifice (called a stenotic gap) is used for the differential diagnosis of visual acuity reduction as an opening stop. The ENT specialist sees through a thin mirror panel of illumination in the neck, nose or ear.

Aperture in photography explained
Influence of the aperture on the exposure and the depth of field


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