|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.
At full aperture, vignetting of a standard lens may be -1 stop at the image periphery, a fairly subtle effect. For extreme wide-angle lenses (diagonal coverage of 85° or more), vignetting may be so strong that it becomes objectionable. Manufacturers of such lenses may offer a correction filter with a graduated neutral density central spot. This reduces the on-axis image illumination to match that in the corners.
Lens designers identify two forms of vignetting: optical & mechanical (also called natural and artificial vignetting). It needs to be said that lens designers often deliberately introduce vignetting for size/weight savings and/or better balance of other aberrations.
A lens illuminates the image plane most brightly at the point where the optical axis intersects it (i.e. image centre). In any lens system, towards the edges illumination falls off for purely optical reasons:
- Light travelling through the centre of the lens passes through the elements at right angles. Light travelling through the lens obliquely travels a longer distance to the image plane and angled through the elements, causing attenuation.
- Light striking the edges of the image plane at an angle causes loss of illumination.
- Finally, light travelling through the lens centre passes through a circular aperture. Off-axis light passes through the aperture obliquely making its shape an ellipse of reduced area.
This "natural" vignetting is sometimes termed Cos4 vignetting, for its trigonometric description. As light exits a lens at off-axis angles, these effects inevitably increase. Wide-angle lenses are most affected, especially since the rear element is close to the film plane. Retrofocus wide angle lenses, with their greater rear distance, reduce vignetting compared to standard designs. Other specific lens design approaches can help improve off-axis illumination. A rough rule of thumb to describe vignetting for non-symmetrical lenses is Cos x*Cos3y where x is angle in object space and y is angle in image space.
Mechanical vignetting is due to physical obstruction of the light path by lens construction, or added filters or hoods. As seen from the corners of the frame, the clear opening of the lens appears clipped into a cat-eye shape. (Besides the loss of illumination, this may also affect bokeh.) Artificial vignetting can be avoided (e.g with larger-diameter lens elements), however many wide angle lenses are deliberately designed with slight artificial vignetting to reduce optical aberrations.
Occurs in digital sensors. Microlenses put above photodiodes increase sensitivity (angular response) in some direction (usually perpendicular to sensor), yet might make it worse for very deviant directions. Sensors with smaller pixels are especially prone, esp. when used with lenses with exit pupil close to sensor. Using thinner manufacturing technology and/or BSI reduces this problem.
- Puts, Erwin. Leica Lens Compendium Hove Books, 2001. Page 80. ISBN 1-897802-17-X