Predicting the effects of defocus blur on contrast sensitivity with a model-based metric of retinal image quality

Abstract

We measure the effect of defocus blur on contrast sensitivity with Sloan letters in the 0.75–2.00 arc min range of letter gaps. We compare our results with the prediction of the Dalimier and Dainty model [J. Opt. Soc. Am. A 25, 2078 (2008)JOAOD60740-323210.1364/JOSAA.25.002078] and propose a new metric of retinal image quality that we define as the model limit for very small letters. The contrast sensitivity is measured for computationally blurred Sloan letters (0, 0.25, and 0.50 diopters for a 3 mm pupil) of different sizes (20/40 to 20/15 visual acuity), and subjects look through a small (2 mm) diaphragm to limit the impact of their own aberration on measurements. Measurements and model predictions, which are normalized by the blur-free condition, weakly depend on letter size and are in good agreement with our metric of retinal image quality. Our metric relates two approaches of modeling visual performance: complete modeling of the optotype classification task and calculation of retinal image quality with a descriptive metric.

Defining metrics of visual acuity from theoretical models of observers

“…In the limit of small Sloan letters, the spectrum can be approximated by a constant function that extends over the full domain of spatial frequency (and therefore does not depend either on ( f x , f y ) or on a ). We had used this approximation to define a model-based metric, M , which predicts contrast sensitivity changes with optical aberrations ( Leroux et al., 2022 ). We obtain the approximated form of S *, which is a quadratic function of letter gap a : …”

“…The letters lost ( Equation 11 ) can therefore be approximated as The argument of the logarithm in Equation 19 is the square of the M metric, which we defined to predict ratios (with/without aberration) of contrast sensitivity measurements from the model of the ideal observer ( Leroux et al., 2022 ; Leroux et al., 2023 ): …”

“…The main advantage is to predict ratios (with/without aberrations) of visual performance measurements without needing a posteriori scale or offset. In our previous work, we reckoned that the Dalimier and Dainty model could be further simplified using the “small letter approximation” to define a model-based metric of contrast sensitivity, M , which we computed directly from optical aberrations without actually simulating the visual images ( Leroux, Fontvieille, Leahy, Marc, & Bardin, 2022 ). We described this metric as model based, as it inherited from the Dalimier and Dainty model.…”