David Jacobson:

Q22. What are aberrations?

A. Aberrations are image defects that result from limitations in the
way lenses can be designed. Better lenses have smaller aberrations,
but aberrations can never be completely eliminated, just reduced.

The classic aberrations are:

* Spherical aberration. Light passing through the edge of the lens is
focused at a different distance (closer in simple lenses) than light
striking the lens near the center.

* Coma. The distance from the axis at which an off-axis object point
is rendered varies with the distance from the center of the lens at
which the light passes. In other words, magnification varies with the
distance from the center of the lens. Off axis points are rendered
with tails, reminiscent of comets, hence the name.

* Astigmatism. Off-axis points are blurred in their the radial or
tangential direction, and focusing can reduce one at the expense
of the other, but cannot bring both into focus at the same time.
(Optometrists apply the word "astigmatism" to a defect in the human
eye that causes *on-axis* points to be blurred along one axis or at 90
degrees to that axis. That astigmatism is not quite the same as
astigmatism in photographic lenses.)


* Curvature of field. Points in a plane get focused sharply on a
curved surface, rather than a plane (the film). Or equivalently, the
set of points in the subject space that are sharp makes a curved
surface rather than a plane. With a plane subject or a subject at
infinite distance the net effect is that when the center is in focus
the edges are out of focus, and if the edges are in focus the center
is out of focus.

* Distortion (pincushion and barrel). The image of a square object
has sides that curve in or out. (This should not be confused with the
natural perspective effects that become particularly noticeable with
wide angle lenses.) This happens because the magnification is not a
constant, but rather varies with the angle from the axis.

* Chromatic aberration. The position (forward and back) of sharp focus
varies with the wavelength.

* Lateral color. The magnification varies with wavelength.


Q23. Can I eliminate these aberrations by stopping down the lens?

A. The effect of all aberrations except distortion and lateral color
is reduced by stopping down. The amount of field curvature is not
affected by stopping down, but its effect on the film is.