We demonstrate that certain combinations of non-rotationally symmetric aberrations (
coma and
astigmatism) can improve
retinal image quality over the condition with the same amount of
astigmatism alone. Simulations of the
retinal image quality in terms of Strehl Ratio, and measurements of Visual Acuity under controlled aberrations with adaptive optics were performed, varying defocus,
astigmatism and
coma.
Astigmatism ranged between 0 and 1.5D. Defocus ranged typically between -1 and 1D. The amount of
coma producing best
retinal image quality (for a given relative angle between
astigmatism and
coma) was computed and the amount was found to be different from zero in all cases (except for 0D of
astigmatism). For example, for a 6mm pupil, in the presence of 0.5D of
astigmatism, a value of
coma of 0.23mum produced (for best focus) a peak improvement in Strehl Ratio by
a factor of 1.7, over having 0.5D of
astigmatism alone. The improvement holds over a range of >1.5D of defocus and peak improvements were found for amounts of
coma ranging from 0.15mum to 0.35mum. We measured VA under corrected high order aberrations,
astigmatism alone (0.5D) and
astigmatism in combination with
coma (0.23mum), with and without adaptive optics correction of all the other aberrations, in two subjects. We found that the combination of
coma with
astigmatism improved decimal VA by
a factor of 1.28 (28%) and 1.47 (47%) in both subjects, over VA with
astigmatism alone when all the rest of aberrations were corrected. Nevertheless, in the presence of typical normal levels of
HOA the effect of the
coma/
astigmatism interaction is considerably diminished.