Vignetting of an image is the fall off from the centre of the image to the outside, usually of brightness but sometimes saturation. It comes about due to some equipment limitations or by design, as a creative effect.
Today it's mostly a creative effect added with filters before of after taking the image. Sometimes it comes about by accident, for example adding a filter or two to some lenses that pushes out their supplied lens hoods to the point where vignetting is experienced.
With some lenses such the popular Nikon AFS Nikor 18-200mm DX G ED VR zoom lens, any filter added to its filter ring produces vignetting at its maximum wide angle (18). This can be overcome by taking off the supplied hood, put on a filter stepping ring, so that a larger sized filter can be used. Alternatively a filter holder and filters system can be used. Without this you cannot use a protective/UV filter on this lens and go to wide angle.
Vignetting may be introduced just for effect, as often with a wedding shot of a bride and groom, as a feature of a reproduction old photo, and more subtlety to focus on the main feature of the photograph.
Some vignetting is present with most lenses, the light through the centre of the image being brighter than the outside, and this is more prevalent with simpler lenses. It is also a feature of some very early lenses and photography.
Historically quite a lot of cameras produced noticeable vignetting.
Modern lenses are designed to reduce this effect, but it is often still present, noticeable at some aperture settings more than others, usually when fully open at its maximum aperture. Generally the faster the speed setting and higher the contrast the more it becomes noticeable.
Where the photographer has it but does not understand the cause its often a combination of various effects that happen to work together, or they have either added two many filters or have mixed up the lens hoods for various lenses.
Vignetting is usually darker towards the outside,
but with some old photos, it is lighter.
There are six causes or artistic scope for Vignetting;-
This is where it is added by the photograph, either in digital editing, by dodging and burning in the wet darkroom, or by the introduction of special effect filters to the camera lens when the image is taken. Understanding the other causes and experimenting allows some photographers the scope to introduce it creatively based on these other methods, but today most add it either in editing or by using an effect filter. Effect filters vary in design, and by changing the aperture and the focal length of a zoom lenses the effect produced by these filters is altered.
Mechanical vignetting occurs when light beams emanating from object points located off-axis are partially blocked by external objects such as thick or stacked filters, secondary lenses, and improper lens hoods. This has the effect of changing the entrance pupil shape as a function of angle. The darkening can be gradual or abrupt, depending on the lens aperture. The smaller the aperture, the more abrupt the vignetting as a function of angle. Complete blackening is possible with mechanical vignetting (when the corner of the image is essentially imaging the inside of the lens hood or filter holder).
This type of vignetting is caused by the physical dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light. The result is a gradual decrease in light intensity towards the image periphery. Optical vignetting is sensitive to the lens aperture and can be completely cured by a reduction in aperture of 2–3 stops. (An increase in the F-number.)
Unlike the previous types, natural vignetting (also known as natural illumination falloff) is not due to the blocking of light rays. The falloff is approximated by the cos4 or "cosine fourth" law of illumination falloff. Here, the light falloff is proportional to the fourth power of the cosine of the angle at which the light impinges on the film or sensor array. Wideangle rangefinder designs and the lens designs used in compact cameras are particularly prone to natural vignetting. Telephoto lenses, retro-focus wideangle lenses used on SLR cameras, and telecentric designs in general are less troubled by natural vignetting.
Pixel vignetting only affects digital cameras and is caused by angle dependence of the digital sensors. Light incident on the sensor at a right angle produces a stronger signal than light hitting it at an oblique angle. Most digital cameras use built-in image processing to compensate for optical vignetting and pixel vignetting when converting raw sensor data to standard image formats such as JPEG or TIFF. The use of offset microlenses over the image sensor can also reduce the effect of pixel vignetting.
Photographic Film Vignetting
This is similar to pixel vignetting in that it is connected with the angle of the light striking the surface. This may be connected with the sensitive layer in the material, refraction of the supporting material or some light may be reflected away.
Extreme Vignetting created in editing to focus the attention of the shot.
Effect filters - special effects like stars, soft effects , netting and more.