Reference Guide

page 9
LENS ABERRATIONS
There is always a difference between the performance of an ideal
lens and an actual lens. This difference is called lens
aberration. There are many types of lens aberrations, but they
fall into two distinct categories. Chromatic aberrations are
caused by differences in the colors of light. Spherical
aberration, astigmatism, coma, distortion, and curvature of field
occur at all visible wavelengths.
It is very important that photographic lenses compensate for
aberrations because the focused image is the key to the picture's
image definition. Good lenses provide well balanced compensation
for aberrations. The image quality of each lens depends on the
amount of aberration remaining after compensation.
There are two kinds of chromatic aberration: longitudinal and
lateral. Longitudinal chromatic aberration is also called
dispersion and is caused by the different indices of refraction
for each color of light. As a result, the image lacks sharpness
and in color photography, there is a fringing effect. In lateral
chromatic aberration, image magnification changes with wavelength,
producing a blurred image similar to the type caused by
longitudinal chromatic aberration. Stopping down the lens has
only a limited effect on these aberrations.
Spherical aberration is caused because the lens is round and the
film is flat. Light entering the edge of the lens is more
severely refracted than light entering the center of the lens.
This results in a blurred image, and also causes flare (non-image
forming internal reflections). Stopping down the lens minimizes
spherical aberration and flare, but introduces diffraction.
Astigmatism in a lens causes a point in the subject to be
reproduced as a line in the image. The effect becomes worse
towards the corner of the image. Stopping down the lens has very
little effect.
Coma in a lens causes a circular shape in the subject to be
reproduced as an oval shape in the image. Stopping down the lens
has almost no effect.
Distortion, whether pincushion or barrel type, is caused by
differences in magnification from the center toward the edges
of the image. Stopping down the lens has no effect at all.
Curvature of Field is the inability of the lens to produce a flat
image of a flat subject. The image is formed instead on a curved
surface. If the center of the image is in focus, the edges are
out of focus and vice versa. Stopping down the lens has a limited
effect.
All Canon EF lenses are very highly corrected for these
aberrations, resulting in unsurpassed image quality. In
particular, we are well-known for extensive use of aspherical
lenses, floating element design, Calcium Fluorite and Ultra-Low
Dispersion Glass, especially in the professional L-Series.