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. In severe cases, the image may approach a circle in shape. The cause can be a lens with insufficient coverage for the image format, or an obstruction to the lens, 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.
All lenses suffer from vignetting which is worse for wide angle lenses. 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 vignetting.
Secondly, light falling on the film or sensor has further to travel at the edges, once again causing fall off.
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, further emphasised 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 lens aberrations.
