Vignetting
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| Example of vignetting produced by a Holga camera image by Voxphoto (Image rights) |
Vignetting or corner shading is an optical effect where illumination falls off towards the periphery of the image. All lenses suffer from vignetting which is an optical characteristic that increases with angle of view. In severe cases, the image may approach a circle in shape. Vignetting can be worsened by a lens with insufficient coverage for the image format, or an obstruction such as a too-small lens hood. It may also be done deliberately for effect - either by using a cut mask in front of the lens, or using image-processing software.
Lens designers indentify two forms of vignetting: natural and artificial.
Natural vignetting
Natural vignetting is inherent in any lens. Light travelling through the centre of the lens passes through the lens elements at right angles. Light travelling through the lens edges passes through at an angle and therefore has a longer path through the lens elements, causing light loss since glass is not perfectly clear.
Secondly, light falling on the film or sensor has further to travel at the edges, once again causing fall off due to dissipation.
Finally, light travelling through the lens centre passes through a circular aperture. Light at the edges, due to the angle, passes through an effective oval aperture of reduced area.
All these factors cause vignetting that cannot be avoided and is present in every lens and more so in wide angle lenses. Natural vignetting worsens the closer the rear element is to the film / sensor plane and retrofocus wide angle lenses, with their greater rear distance, reduce vignetting compared to standard designs.
Artificial vignetting
Artificial vignetting is caused by lens construction, or added filters or hoods, physically blocking the light path. Artificial vignetting can be avoided, however many wide angle lenses are deliberately designed with slight artificial vignetting to reduce optical aberrations.
References
- Puts, Erwin. Leica Lens Compendium Hove Books, 2001. ISBN 1-897802-17-X
