Vignetting

Glossary Terms

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: optical & mechanical (also called natural and artificial vignetting).

Optical vignetting

A lens illuminates the image plane most brightly at the point where the optical axis intersects it. Moving outwards from this, illumination falls off simply for geometric reasons:

• Light travelling through the centre of the lens passes through the lens elements at right angles. Light travelling through the lens obliquely passes travels a longer distance to the image plane. From the corners of the image frame, the clear aperture of the lens appears smaller.

• Light striking the edges of the image at an angle also causes a loss of illumination.

• Finally, light travelling through the lens centre passes through a circular aperture. Moving outwards, the aperture shape increasingly appears as an ellipse of reduced area.

This "natural" vignetting is sometimes termed Cos⁴ vignetting, for its trigonometric description. As light exits a lens at steeper angles, these effects inevitably increase. Thus wide-angle lenses are most greatly affected. 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.

Mechanical vignetting

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.

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.

References

1. Puts, Erwin. Leica Lens Compendium Hove Books, 2001. Page 80. ISBN 1-897802-17-X

Links

• "Covering Power of Lenses" in The Focal Encyclopedia of Photography, 3rd Edition, by Leslie Stroebel, Richard D. Zakia (Focal Press, 1993) pages 422–423; from Google Books.