Effect of Target Luminance on Optimum Pupil Diameter for Presbyopic Eyes

To quantify the optimum pupil diameters for presbyopic eyes when environmental light levels vary from high photopic to low mesopic.

We computed the white light visual Strehl ratio from the polychromatic optical transfer function (VSOTF). The impact on image quality of retinal illuminance and the accompanying changes in photon noise were reflected in changing neural contrast sensitivity. VSOTF was calculated for focused and -2D defocused images, over a wide range of photopic and mesopic target luminances, and for pupil diameters ranging from 1 to 7 mm. We compare these modeling data to logMAR visual acuity measured under the same conditions.

Optimum pupil depends on the relative gain of optical factors (diffraction and aberrations) and neural thresholds (photon noise effect). When light levels are reduced from 1000 to 2 cd/m, the pupil size that optimizes VSOTF for a well-focused presbyopic eye model also maximizes visual acuity and it increases from 2.5 to 4 mm. A 1-mm-diameter pupil maximizes VSOTF at all light levels when -2D of defocus are included, but at 2 cd/m the optimum pupil diameter for acuity increases slightly to 1.4 mm. At 0.2 cd/m, the pupil size that optimizes VSOTF remains 4 and 1 mm for a focused and -2D defocused eye, respectively, but significantly larger pupils are needed for maximal visual acuity. Reducing pupils to 30% of their natural size is beneficial for distance and near image quality and visual acuity at all light levels, producing more gains for near than for distance.

A fixed 2- to 3-mm small pupil or a 30% pupil miosis can both produce near visual acuity gains without significant losses to distance acuity or image quality, and therefore can be considered as optimal for a presbyope experiencing a wide range of light levels.