Difference between revisions of "Vignetting"
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All these factors cause vignetting that cannot be avoided and is present in every lens and more so in wide angle lenses. Optical 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. | All these factors cause vignetting that cannot be avoided and is present in every lens and more so in wide angle lenses. Optical 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. | ||
| − | Typical vignetting of a standard lens is -1 stop at image periphery. | + | Typical vignetting of a standard lens is -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 filter|neutral density]] central spot. This reduces the on-axis image illumination to match that in the corners. |
===Mechanical vignetting=== | ===Mechanical vignetting=== | ||
Revision as of 17:48, 23 March 2012
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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: optical & mechanical (also called natural and artificial vignetting).
Optical vignetting
Optical 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 due to dispersion.
Light falling at an angle at the edges of the image also causes dispersion.
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. Optical 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.
Typical vignetting of a standard lens is -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.
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.
References
- 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.
