Astigmatism, accommodation, the oblique effect and meridional amblyopia.

The variation with accommodation in the dimensions of the geometrical blur ellipse that is formed on the retina of the astigmatic eye is explored in relation to orientation dependence of the corresponding line-spread function (LSF) and modulation transfer function (MTF). It is shown that if theta is the orientation of the line or grating with respect to the more powerful meridian of an eye having C dioptres of astigmatism, the optimal accommodation response varies as 0.5 C cos2 theta. The analysis is extended to include the effects of diffraction. Even at optimal focus, the diffraction-limited MTF at 45 degrees to the principal meridians of an eye with small amounts of astigmatism is substantially degraded. For example, with a 3 mm pupil and only 0.50 DC of astigmatism, the first zero in monochromatic modulation transfer at 555 nm occurs at approximately 32 c/deg in the oblique meridian as compared with 94 c/deg in the principal meridians. The results are used as the basis of a discussion of the possible role of infant and juvenile astigmatism in the development of the oblique effect and of meridional amblyopia.