Vignetting

As vignetting (French vignette " border decoration ", " badge " ) is called in the photographic shading towards the margins, which is caused by an axial arrangement of two openings. You can also occur in reflecting telescopes and is bypassed by increasing the primary mirror at the Schmidt telescope.

In the photographic practice, the term is often incorrectly interpreted somewhat generous and also used for the light falloff.

In general, vignetting and light falloff unwanted effects that are to be avoided. Often photographers use the resulting darkening towards the edge but also deliberately to emphasize the center of the image or to change the mood of the recording. The effect can also be added later by a digital filter.

• 2.1 Natural Randlichtabfall
• 2.2 Randlichtabfall by inadequate lighting

True vignetting

Optical vignetting

The cause of the optical vignetting is that the light rays impinging upon the image plane, both the inlet opening (eg, a lens ) and the discharge opening (and possibly additional openings, such as lenses or diaphragms ) must pass. At a certain distance from the optical axis "sees" a film in the image plane through the exit aperture, only the edge of the inlet opening, at even longer intervals, the inlet opening is not to be seen - and, therefore, no light on the image plane there is also. meet In other words, the darkening is the ( fuzzy ) Shadow of the outlet.

The animation illustrates these relationships:

It may here give the impression that a panel for the vignetting is responsible. In fact, the same relationships apply if, instead of an orifice a second lens in the beam path ( such as in a telescope ). The beams are additionally bundled or fanned depending on the type of lens.

Below this is the example of a so-called telephoto designs illustrated ( telephoto design refers to a design principle that is often used to shorten the overall length of long focal length lenses). This results in vignetting by the same the aperture of the exit lens as an aperture:

The optical vignetting can be reduced by stopping down, possibly even to the extent that the entire image is completely free of optical vignetting. At first glance, it seems amazing that you can reduce a blackout by yet another " shadowing " by the aperture. In fact, it increased by stopping down the vignetting area, but you also simultaneously reduced by the maximum usable area. The zone of vignetting becomes narrower.

It is evident that increases by stopping the vignetting image circle. This is important, for example, in the large-format photography. Many lenses for large format allow to use by stopping a larger image circle. But normal small -screen or compact camera lenses show at open aperture a visible vignetting, which may need to be reduced by stopping down.

Conversely, this also means that lenses for small recording formats ( Crop - sensor, eg APS -C) can be built at a comparable cost of materials with a larger maximum aperture (and therefore brighter ) without thereby stronger vignetting as lenses for larger recording formats.

Maximum luminance

With telephoto lenses, the maximum aperture may be opened so wide and the telephoto member so great that the vignetting starts in the center of the image, the vignetting zone thus completely disappears. The zone of vignetting is so wide that the brightness decreases only slowly. The film "sees" in the center of the image through the exit opening, the whole entry lens. A further enlarge the outlet opening would not brighten the picture in the center therefore on. The luminance in the center of the image would not get any bigger, while on the other hand, the photographic light intensity would increase arithmetically on.

Analogies

The geometric relationships do not change when an opening is inverted, so to speak, and the original "hole" is replaced by an opaque disc. The vignetting zone becomes the umbra, the zone of vignetting becomes the penumbra and the completely darkened zone to completely shadow-free zone. Vignetting can be considered such as a solar eclipse geometrically so.

Mechanical vignetting

Shadows - sometimes referred to as mechanical or physical vignetting - are caused for example by incorrectly selected lens hoods or too many filters or lens attachments, their sockets dim the marginal rays. It has basically nothing to do with the lens design when mounted on the lens objects - or their own fingers - accidentally fall into the picture when recording.

Pixel vignetting

Pixel vignetting is directed to a Randlichtabfall the only digital recording media. He comes from the fact that the individual photosensitive elements of a digital image sensor does not lie on the surface of the sensor, but rather by design are in tiny wells. Just as the flat light rays of the evening sun no longer extend into the valleys, and flat incident light beams can illuminate the sensitive area of ​​the photodiode only partially. The Pixel vignetting can be improved technically in principle, eg by use of microlenses. Modern image sensors compensate for the pixel vignetting by appropriate algorithms already electronically " OnChip ".

Randlichtabfall

When Randlichtabfall not it is actually a vignetting because the cause of the light falloff is no shading.

Natural Randlichtabfall

The natural light falloff - often mistakenly called natural vignetting - leads towards a steady darkening of the image to the image corners. The largest effective aperture of a lens decreases the further the object to be imaged by the optical axis is removed. The amount of light decrease is approximately proportional to the fourth power of the cosine of the angle to the optical axis ( Cos4 Act). The natural vignetting can be eliminated either by stopping or by design measures; it can at best be reduced somewhat by a suitable lens design. This type of vignetting is most striking in the mapping of areas of uniform brightness.

Randlichtabfall by inadequate lighting

Sometimes also Randabdunkelungen by a defective illumination are, for example, erroneously referred to by the electronic flash as vignetting. For example, a flat object a flash lights on or in the camera off unevenly. Here, the parts of the design that are most distant from the light source is weaker than nearer illuminated parts of the scene. Lights up the flash unit from a smaller angle than is imaged by the lens on the recording medium ( see picture angle), this also has a Randabdunkelung result.