Vignetting is the loss of light, usually around the periphery of an image, as a consequence of an incomplete bundle passing through the optical system. A vignetted image appears dimmer around the periphery.
Most binoculars suffer from some degree of vignetting. The exception is some binoculars designed specifically for astronomical use whose construction is based on astronomical refracting telescopes which themselves give unvignetted images.
In some, vignetting can be so severe that no part of the image is illuminated by the complete aperture. In normal daylight use, we do not notice vignetting unless it is exceptionally severe; 30 percent is common and 50 percent is sometimes deemed acceptable in wideangle systems. This is because, at any given time, only a tiny region of the image can be examined by the fovea, it is, therefore, only this region that needs to be fully illuminated. As long as the fall-off of illumination toward the periphery is smooth, it will not normally be noticed.
Binocular astronomers who, like other astronomers, echo the call for "more light!" sometimes wonder why vignetting is allowed to occur at all. To understand this, we must first understand the role of the aperture stop. An aperture stop crops the light cone and eliminates the most peripheral rays. These peripheral rays have the highest angles of incidence on the optical surfaces and undergo the most refractive bending. For these reasons they also carry with them the greatest amount of aberration. If they are permitted to pass through to your eye, they will add to the degradation of the image. Part of the process of good optical design is to assess how much of the peripheral light needs to be excluded.
If bundles of rays from all parts of the field of view fill the aperture stop, then there is no vignetting. On the other hand, if some other mechanical or optical component impedes some of this light, vignetting will occur. An unvignetted binocular requires larger optical apertures all the way through the optical system when compared to one in which vignetting does not occur. This in turn requires larger optical components (such as prisms or focusing lenses). Larger components are not only more expensive, but are also heavier. Heavier components require heavier and more robust mountings. These in turn add to the expense of the binoculars. The overall result is a heavier, more expensive, binocular. In short, vignetted systems are usually smaller, lighter, and produce better images in comparison to the equivalent unvignetted optical system. Somewhere in the design process a decision is made as to where an acceptable trade-off lies. The more discerning observer may well be prepared to accept a more expensive instrument, but the general user will almost certainly not want to pay considerably more for a hardly noticeable increase in light throughput at the periphery. Even the discerning observer may balk at an increase in weight if the binoculars are intended to be hand held.
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