
84 5. Fermat's Principle and Abberrations
AQ
> 0 the reimaged square resembles a pincushion; ifAQ<0 the result is barrel
distortion. See Bom and Wolf
(1980),
or Welford (1986), for schematic diagrams
of distortion.
For a mirror the paraxial equation is exact, and hence there is no distortion
when the stop is at the surface. Distortion generally is present when the stop is
displaced from the surface of a mirror.
Finally, we have not yet considered curvature of field, the final aberration
noted in Section 4.4, Characteristics of
this
aberration are given in Section 5.7 of
this chapter.
For more details on the nature of aberrations and wavefron ...