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88 Chapter Five
Figure 5.15 The variation of the aberrations with lens shape when the
stop is located in the “natural” (coma free) position for each shape. Data
are for 100-mm f/10 lens covering 17° field, made from BSC-2 glass
(517:645).
aberrations of the lens have again been plotted against the lens shape;
however, in this figure, the aberration values are those which occur
when the stop is in the natural position. Thus, for each bending the
coma has been removed by choosing this stop position, and the field is
as far backward curving as possible.
Notice that the shape which produces minimum spherical aberration
also produces the maximum field curvature, so that this shape, which
gives the best image near the axis, is not suitable for wide field coverage.
The meniscus shapes at either side of the figure represent a much better
choice for a wide field, for although the spherical aberration is much
larger at these bendings, the field is much more nearly flat. This is the
type of lens used in inexpensive cameras at speeds of f/11 or f/16.
5.5 Aberration Variation with Aperture
and Field
In the preceding section, we considered the effect of lens shape and
aperture position on the aberrations of a simple lens, and in that
discussion we assumed that the lens operated at a fixed aperture of
