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Vignetting (V) is a widespread photographic problem. Most lenses and certainly all compact cameras show it. It is most visible at the corners as a radial loss of brightness, due to lowered exposure.Vignetting can be used as an effect to draw attention to the central subject of a picture, but in general it is undesirable. Vignetting actually has two aspects: The common underexposed corners and the bright center, called the hot spot.
There are several causes of vignetting: 1. Mechanical. 2. Optical. 3. Natural. 4. Pixel.
Mechanical vignetting can be caused by three things: too many filters stacked on the lens, too small a lens hood attached to the lens; or secondary lenses. This will reduce the light at the corners and darken them. Depending on the lens aperture, the darkening will be abrupt or gradual. The smaller the aperture, the more abrupt the vignetting.
Optical vignetting is inseparable from the lens itself and is caused partly by the combination of several lens elements, partly by the fact that the lens itself has a considerable length. Optical vignetting is also called artificial vignetting.The length of the lens is a problem because when the lenses are used with a wide open aperture, the edges of the lens will occlude part of the aperture from oblique incoming light, thus shading the edges. Zoom lenses and wide angle lenses are particularly prone to this kind of V. The rear elements of the lens are shaded by the elements in front of them reducing the light radially and causing vignetting. One can often cure optical V by reducing the aperture two or three stops. This kind of vignetting can be reduced by using very large front lens elements and this is often done in wide angle lenses. Lastly, the contrast of the film or sensor plays a part: the stronger the contrast, the more pronounced the vignetting.
Natural vignetting (also called natural light falloff) is a natural light falloff proportional to the angle the light reaches the sensor or film; it is not caused by the lens. Technically the falloff is proportional to the fourth power of the cosine of the angle of the in-falling light on the film or sensor. Lenses in compact cameras are particularly prone to such falloff. So are wide angle lenses. Telephoto lenses show the least falloff. At large apertures both optical and natural vignetting are present. The combined effect is often called illumination falloff or radial density.
Pixel vignetting is of course not relevant for film, but only for digital cameras. It is created because most sensors have an angle dependency of the in-falling light.Light hitting the sensor at a right angle produces a stronger impression than light hitting at an oblique angle, thus the corners become underexposed. Digital cameras often have a built-in compensation for this, used when converting the RAW image data to tiff or jpg. If one works with RAW images, one will have to do post-processing to remove pixel V.
This article is based on this one Vignetting in Photos.