Vignetting
The fall-off of image brightness toward the edge of the field. Mechanical vignetting comes from lens barrels and apertures clipping off-axis ray bundles; natural vignetting follows roughly cos⁴θ even in a perfect lens, dropping to 78% at 20° and 56% at 30° off axis.
A uniformly lit scene rarely produces a uniformly bright image: the corners are darker than the centre. That shading is vignetting, and it has two main causes.
Mechanical vignetting
In a lens with several elements, the full aperture is available on axis, but a bundle of rays from an off-axis point meets the front and rear elements, or the lens barrel and mounts, at different heights and is partly clipped. The effective aperture seen from the edge of the field shrinks to a lens-shaped overlap of two circles, and the image dims. Stopping the lens down, to a larger f-number, reduces this kind of vignetting, because the smaller aperture stop then limits every bundle before the other apertures do. Designers sometimes accept it deliberately, since the clipped marginal rays at the edge of the field carry the worst aberrations. Adding a filter, hood or adapter that the design did not allow for is the common bench cause.
Natural vignetting
Even with no clipping, image irradiance from a uniform, Lambertian scene falls off with field angle approximately as
one factor of from the foreshortening of the pupil seen obliquely, two from its greater distance (inverse square), and one from the obliquity of the image plane. The law gives 94% at 10°, 78% at 20° and 56% at 30°, so a wide-angle lens loses about half its light at the edge of a 60° field from geometry alone. Well corrected wide-angle designs reduce it by making the pupil appear larger off axis; telecentric lenses, where the chief rays are parallel to the axis in image space, largely avoid the image-side part. Pixel-level effects in camera sensors add their own angle-dependent losses.
Why it matters
In photography vignetting is often tolerated or added for effect. In measurement it biases results: a beam profile, fluorescence image or spectral map taken through a vignetting system reads low at the edges. In laser scanning and illumination optics it shows as nonuniform power across the field, and in fiber coupling and spectrometers an overfilled or clipped aperture shows up as unexpected loss.
Measurement and correction
Vignetting is measured by imaging a uniform source, an integrating sphere port or a well diffused flat panel, and mapping the image brightness against field position. The resulting flat-field image is used to correct later images by division, which is standard practice in quantitative microscopy and astronomy.
References: W. J. Smith, Modern Optical Engineering, 4th ed. (McGraw-Hill, 2008), Ch. 6; R. Kingslake, Optical System Design (Academic Press, 1983).