Vignetting is a widespread photographic issue. It is found in most lenses and in all cheap compact cameras. It is a radial loss of brightness (due to exposure drop) visible at the corners. Vignetting is generally undesirable, but it can be used to draw attention to the central subject of the picture. There are in fact two aspect to vignetting: 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 is typically caused by too thick a stack of filters, or a lens hood attached to the lens, or by secondary lenses. All three may reduce the light at the corners and darken them. The darkening will be abrupt or gradual depending on the aperture. The smaller the aperture, the more abrupt the vignetting.
Optical vignetting is inseparable from the lens makeup itself and is caused partly by the combination of several lens elements, partly by the fact that the lens itself has a considerable length. It is also called artificial vignetting. When lenses are used with a wide open aperture, the length of the lens becomes a problem: the edges of the lens will shadow the edges of the aperture and thus darken the edges of the photo. This kind of V is most pronounced in zoom lenses and wide angle lenses. The light in the lens is also reduced radially because the rear lens elements are shaded by the front lens elements, thus causing vignetting. One can often cure optical V by reducing the aperture two or three stops. Very large front lens elements tend to reduce this kind of vignetting and are typically used in wide angle lenses. Finally should be mentioned that the contrast of the sensor or film also plays a part: the stronger the contrast, the stronger the vignetting.
Natural vignetting (also called natural light falloff) is a, well, 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 will be 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 reaching the sensor at an oblique angle generates a weaker sensor reaction than light reaching it at a right 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 jpeg. If one works with RAW images, one will have to do post-processing to remove pixel V.