Combining coma with astigmatism can improve retinal image over astigmatism alone

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 1.5 D. Defocus ranged typically between −1 and 1 D. 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 0 D of astigmatism). For example, for a 6 mm pupil, in the presence of 0.5 D of astigmatism, a value of coma of 0.23 μm produced (for best focus) a peak improvement in Strehl Ratio by a factor of 1.7, over having 0.5 D of astigmatism alone. The improvement holds over a range of >1.5 D of defocus and peak improvements were found for amounts of coma ranging from 0.15 μm to 0.35 μm. We measured VA under corrected high order aberrations, astigmatism alone (0.5 D) and astigmatism in combination with coma (0.23 μm), 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.