Effect of instrumental spherical aberration on visual image quality

The effect of instrumental spherical aberration has been examined through the construction of telescopes with approximately 1, 2, and 3 waves of aberration. Contrast sensitivity and resolution results have been obtained, as well as information on the preferred focus in the presence of aberration. The results show that an appropriately defined integral of the instrumental modulation transfer function (MTF), called MTFa, is the best candidate among objective image quality metrics for describing the observed degradation in performance as well as the preferred focus. For pupil diameters less than approximately 3 mm the results support the assertion that the monochromatic aberrations of a normal eye are of little importance in visual instrument optical design and testing.