Since 1/u'2 = (f'/yi)2 and the coefficients Sm and Sjv always contain the factor (yi/f')2 (see Tables 3.4 and 3.5), we see that these longitudinal measures are independent of aperture. If we now set y = ym for the edge of the pupil, then:


Ipr Im

At the mean astigmatic focus Im, the image diameter is given by

5nq ast,m

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