Any failure of a lens to meet theoretical
standards of perfection is termed an aberration. Virtually no lens is
entirely free of some residual aberrations, no matter what the effort
to achieve perfection. In lens design and manufacture, as in most other
aspects of photography, there is constant compromise, as one thing is
traded off to improve another. For instance, lens speed is often
obtained at the expense of flatness of field or freedom from
distortion, which may also have to be sacrificed to obtain width of
coverage in wide angle lenses. Nearly any economy of manufacture
requires some departure from perfection in the lens system. Further,
any demand for unusual perfection in a single optical feature tends to
require some compromise elsewhere in the design.
Although, with the sophisticated computer
technology in lens design, and the increasing use of new types of glass
in manufacture, most modern lenses are amazingly good. Lenses that give
genuinely poor performance are becoming increasingly rare. However, it
is useful to know a little about lens aberrations in case you run
across the unusual in practice.
Some types of aberrations have more
effect or have more obvious effects on photographic images than others.
Some are more likely to appear, given the manufacturing methods of the
day and the design emphases that are most common. Some of the most
common aberrations for camera lenses nowadays are astigmatism,
chromatic aberration, coma, curvature of field, diffraction,
distortion, flare, spherical aberration, vignetting.