Optical aspects of tolerances to uncorrected ocular astigmatism.

We approached the question of the visual tolerance to uncorrected astigmatism by exploring the changes in the retinal diffraction images brought about by astigmatism, as a function of spherical focus. Consideration of the Rayleigh and Maréchal wavefront criteria and of the changes in ocular modulation transfer function (MTF) broadly supports a value of 0.25 DC as a practical tolerance limit for uncorrected astigmatism. This study underlines the limitations of a geometrical optical approach to this problem in particular, and shows that the concept of the circle of least confusion may be misleading.