Vignetting
As vignetting ( fr. Vignette "border ornament", "badges") is known in the photographic technology shadowing to the image towards the edge (vignetting), by a axial caused arrangement of two or more openings. It can also occur with reflector telescopes and is circumvented in the Schmidt telescope by enlarging the main mirror.
In general, edge light falloff is an unwanted effect. Sometimes, however, photographers use the resulting darkening towards the edge intentionally to emphasize the center of the picture or to change the mood of the shot. The effect can also be added later during digital image processing .
Real vignetting
Construction-related vignetting
(also: optical vignetting )
It arises from the fact that the light rays have to pass through several successive openings ( lens edges , diaphragms ) before they reach the image plane. A bundle of rays symmetrical to the optical axis (starting from an object point on the axis) is delimited by one of these openings (aperture diaphragm; usually an iris diaphragm ) and completely fills it. However, if the rays are incident at an angle to the axis (image of an object point next to the axis), the bundle can also be cut by further openings and no longer completely fills the aperture diaphragm, which means that less light reaches the image plane. This is called vignetting.
The animation illustrates these relationships.
Most camera lenses are deliberately designed with vignetting, as this enables a better compromise to be achieved between light intensity , correction of aberrations , cost and size of the lens. Before and after the aperture diaphragm there are usually further diaphragms which are completely filled by the axially parallel beam when the aperture diaphragm is fully open. Bundles incident at a small angle are then vignetted. As you stop down the lens, vignetting is reduced and there is an area around the center of the image where vignetting no longer occurs. Typically, stopping down two to four f-stops is enough to expand this area to the corners so that the vignetting disappears completely.
In this context there is no difference between a diaphragm and a lens edge. The latter has the same limiting effect on the beam. This is shown below using a telephoto lens as an example ( telephoto refers to a design principle that shortens the length of long focal length lenses). The same vignetting results through the opening of the exit lens as with an aperture.
At first glance, it seems astonishing that darkening can be reduced by further “shading” through the aperture (reducing the aperture). In fact, stopping down increases the vignetting-free area, but this may also reduce the maximum usable area, namely if the outermost, incident beam at the maximum angle, which just barely comes through with the aperture open, is stopped by the reduced aperture. The zone of vignetting becomes narrower as a result.
It can be seen that the vignetting-free image circle is enlarged by stopping down. This is important e.g. B. in large format photography . Many lenses for large format make it possible to use a larger image circle by stopping down (but this is mainly due to the fact that the aberrations are reduced by stopping down and a larger image area becomes sufficiently sharp as a result). But even normal 35mm or compact camera lenses often show visible vignetting when the aperture is open , which can be reduced by stopping down.
Usage-related vignetting
This refers to shading caused by additional parts before or after the lens, for example, incorrectly chosen baffles or too many filters or conversion lenses whose frames block out the peripheral rays. These shadows are sometimes referred to as mechanical or physical vignetting .
Pixel vignetting
Pixel vignetting is a drop in edge light that only affects certain recording media, such as image sensors or microchannel plates . It comes about because the individual light-sensitive elements of a sensor do not lie on the surface, but rather are located in tiny depressions due to the design. Just as the flat rays of light from the evening sun no longer reach into the valleys, even rays of light hitting flat can only partially illuminate the light-sensitive surface of the sensors. The pixel vignetting can be technically improved in principle, for example by using microlenses or minimizing the depressions. Modern image sensors can already electronically compensate for pixel vignetting using appropriate algorithms "on chip " .
Influences on the edge light falloff
A drop in image brightness towards the edge of an image created by optical imaging is influenced by various factors:
- the natural fall off from the edge of light ( cos 4 law ), which is physically conditioned and unavoidable
- the real vignetting caused by the lens or additional components on the lens
- Pixel vignetting, e.g. B. by the properties of a digital image sensor is effected
- Distortion
- Pupillary aberration
- Uneven illumination of the subject, typically caused by flash units with direct flash
The natural edge light falloff produces a falloff that is proportional to the fourth power of the cosine of the angle to the optical axis. It can neither be remedied by dimming nor by constructive measures. This decrease in brightness occurs very evenly from the center to the edge and is therefore hardly noticeable if it is not too strong. It is most clearly noticed when imaging contourless surfaces of uniform brightness (e.g. blue sky).
A barrel-shaped distortion causes the illuminance of the image plane to increase towards the edge. As the image height increases, the image points become denser, since they are shifted towards the center of the image. This reduces the drop in brightness. However, this effect only has a significant effect on fisheye lenses that are strongly barrel-shaped. Conversely, the decrease in brightness is increased by a pincushion distortion.
A pupillary aberration arises from the imaging of the diaphragm onto the entrance pupil . If this occurs with considerable errors , the image of the diaphragm can be distorted depending on the angle of incidence of the bundle of rays, which in turn influences the cross section of the bundle of rays entering and thus the amount of light.
There are several terms in use for natural edge light loss and vignetting (e.g. optical , technical , mechanical , artificial vignetting ) that are not used uniformly. The natural fall off from the edge of light is sometimes incorrectly referred to as natural or physical vignetting , even though it is not vignetting because the cause of the fall off from the edge light is not shadowing.
Occasionally, edge darkening due to inadequate illumination, e.g. B. falsely referred to as vignetting by the electronic flash. For example, a flash unit lights up or a flat object in the camera lights up unevenly. The parts of the subject that are furthest away from the light source are less illuminated than parts of the scene that are closer. If the flash unit illuminates a smaller angle than the lens shows on the recording medium (see angle of view ), this also results in a darkening of the edges.
correction
In many wide-angle lenses , a pupillary aberration is deliberately created in order to enlarge the cross-section of bundles of rays emanating from points adjacent to the axis. This is the main reason why the decrease in brightness in many lenses is less pronounced than it should be according to the cos 4 law.
An uneven distribution of brightness can also be reduced or eliminated using a graduated filter. This is a gray filter that is uncolored on the edge and darkens towards the middle. However, this reduces the effective light intensity. The manufacturers of extreme wide-angle lenses sometimes supply a suitable graduated filter that corrects the decrease in brightness to such an extent that it is no longer bothersome.
With digital post-processing it is easy to influence the decrease in brightness as desired and also to eliminate it completely. However, the disadvantage of a reduced signal-to-noise ratio in the darkened corners remains. By brightening these areas, the sensor noise is more pronounced. When enlarging an analog recording, you can dodge the edges and corners and thereby brighten them.
