Vignetting

vignetting

(1) A defect of an optical system in which light at the edges of images is cut off or reduced. It is caused by an obstruction in its original construction; for example, when the elements used in a lens are too small.

(2) A visual effect of darkened corners used to help frame an image or soften the frame outline.

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vignetting [vin′yed·iŋ]

(optics)

Reduction in intensity of illumination near the edges of an optical instrument's field of view caused by obstruction of light rays by the edge of the aperture.

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Vignetting 

the partial darkening of a bundle of light rays entering an optical system, which is accomplished by restricting the light with the apertures of the instrument. Vignetting leads to a gradual decrease in the illumination of an image in the transition from the center to the edge of a field of vision. It is totally absent only when the plane of the entrance aperture coincides with the plane of the object (correspondingly, when the plane of the exit aperture coincides with the plane of the image). In this case, the image is sharply limited. There is another form of vignetting, achieved in mirror and mirror-lens systems through the use of a second reflecting element, which hinders the dissemination of the central rays.

Vignetting is important in the use of photographic lenses. It usually does not exceed 30-40 percent, but in wide-angle lenses it can reach 50-60 percent. As a result, the edges of the photographic film seem incompletely exposed. Vignetting should be considered in spectrum analysis—for example, when there must be even illumination along the full height of the image of the spectrographic slit.

REFERENCES

Tudorovskii, A. I. Teoriia opticheskikh priborov. Part 1. Moscow-Leningrad, 1948.
Mandel’shtam, S. L. Vvedenie v spektral’nyi analiz. Moscow-Leningrad, 1946.