Where is the optimum far-point for a presbyopic eye?

PURPOSE: To resolve the presbyope's dilemma by determining the optimum location of the far-point that maximizes depth-of-field for reading printed text.

METHODS

Geometrical optical analysis of defocused retinal images was used to compute the size of retinal blur circles relative to object size. Functional consequences of changes in viewing distance, pupil diameter, and far-point location were assessed using the blur ratio concept. Depth-of-field was specified by the ratio of the maximum distance to the minimum distance for which printed text of a given size is legible.

RESULTS

For the emmetropic patient, text that is legible at one viewing distance remains legible at all shorter distances. Conversely, text that is illegible at one distance is illegible at all distances. For myopic (or undercorrected) patients, the location of the far-point determines the center of the depth-of-field, but not its size. The depth-of-field is shown numerically and analytically to be given by the approximate formula: log(far distance/near distance) = 0.174* blur ratio threshold* letter height/pupil diameter.

CONCLUSIONS

Location of the far-point is a free parameter that can be adjusted to suit a patient's needs without affecting depth-of-field. This suggests a theoretically based clinical strategy for presbyopic refractive correction that takes account of reading needs, pupil size, text size, and habitual reading distance for the benefit of the presbyopic patient.