Subjective visual acuity with simulated defocus

Background: Artificial degradation of vision by inducing dioptric blur is frequently used for evaluating diagnostic equipment. However, the use of lenses is prone to errors as it is adversely affected by eyelid squinting, pupil size, and imprecise lens position. The alternative is a degradation of the stimuli themselves based on a Fourier-optical mathematical model. With this, however, perceptual effects such as 'simultaneous blur' induced by the surround may affect acuity. We tested whether both methods, lens induced defocus and mathematical stimulus degradation, yield concordant results. Methods: We compared both methods in normal subjects, measuring Landolt C acuity at five different levels of defocus from 0 to 8 dioptres. The pupil size was determined individually, chromatic aberrations were avoided by using a monochromatic approach, and a correction for spectacle magnification was included. Otherwise, the experimental design was kept deliberately simple to remain comparable to typical applications. Results: With the major sources of error associated with the use of lenses being accounted for, both methods yield similar mean results with differences in acuity ranging from <0.001 to 0.054 logMAR (0.2-13%). Conclusion: Using mathematically simulated defocus is a viable option if both reasonable accuracy and ease of use are required. The fact that the application of the mathematical model only blurs the stimulus itself, but not the surrounding environment, does not appear to be detrimental to the method.