Optimizing the subjective depth-of-focus with combinations of fourth- and sixth-order spherical aberration

We optimize the subjective depth of focus (DoF) with combinations of spherical aberration (SA4) and secondary spherical aberration (SA6) in various levels. Subjective DoF was defined as the visual interval for which three 20/50 high-contrast letters was perceived acceptable (objectionable blur limits). We used an adaptive optics system to dynamically correct the observer's aberrations and control their accommodation. DoF was measured with a 0.18-D step on three non-presbyopic subjects. The target seen by the subjects was modified to include 25 combinations of SA4 and SA6 (i.e. 0, ± 0.15 and ± 0.30 μm) for 3, 4.5 and 6mm of pupil diameter. We found a mean DoF of 1.97D with a 3mm pupil size, which decreased by 28% with a 4.5mm pupil and by 34% with a 6mm pupil. For 6mm pupil we found an increase of subjective DoF of 45% and 64% with the addition of 0.3 and 0.6 μm of SA4, and of 52% and 117% with the addition of 0.15 and 0.3 μm of SA6. The largest DoF measured (4.78D) increased 3.6 times that of the naked eye and was found for a combination of opposite signs of SA4 and SA6 of 0.6 and 0.3 μm respectively. Reducing the pupil size minimized the effect of aberrations on subjective DoF. Combination of SA4 and SA6 of opposite sign could increase DoF more than three times for pupils larger than 4.5mm. Subjective DoF is well predicted by measuring the induced variation of vergence arising in the pupil size.