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What is spherical aberration?
Answer provided by Mike Collins.
An aberration is an optical defect in a lens or mirror. Spherical aberration occurs when rays of light passing through a lens do not come to a focus at a single point. As shown in Figure 1, rays which pass through the outer edges of the lens come to a focus closer to the lens than rays which pass through closer to the center.
Figure 1: Spherical aberration occurs when rays fail to focus to a single point.
Spherical aberration is due to the spherical shape of the lens, hence the name. Spherical aberration is more apparent at wider apertures and the effect is a slight softness in focus known as a “circle of confusion.” The effect is more obvious at the edges of a photograph. Focus also tends to shift as the lens is stopped down from wider apertures to smaller apertures.
Spherical aberration also occurs in so-called mirror lenses, which are also known as catadioptric lenses. Figure 2 is a photograph taken with a Vivitar catadioptric lens:
Figure 2: Blue Flag Irises shot with a Vivitar 500mm catadioptric lens (Shiawassee National Wildlife Refuge).
The entire photograph has a very soft focus due to the level of spherical aberration characteristic of this particular lens.
Most lenses are designed to reduce optical defects such as spherical aberration as much as possible. However, so-called soft focus lenses are designed to retain some spherical aberration for artistic reasons. Canon, Mamiya, and Pentax market soft focus lenses, for example. Soft focus lenses are used in portrait photography, for example, to help reduce the effects of wrinkles and other blemishes.
Although most lenses today are designed using computers to reduce aberrations, as with most things, cheaper lenses are typically going to perform more poorly. Our vice president, Ron St. Germain, recommends testing any lens before purchase to determine whether the quality is satisfactory. Make sure you can return the lens if you are not completely happy. You can find a printable copy of a resolution test chart here.
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