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 ) JOAOD6 0740-3232 10.1364/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.