The Relationship between Defocused MTF and Spatial Frequencies Needed for Letter Recognition

In 1990, Frank Thom and Faye Schwartz published an interesting study of the effects of defocus on letter and sine-wave gratings. By comparing with theoretical estimates of the Modulation Transfer Function (MTF) of the observer's eye, they concluded that "letters are recognizable only if the spatial components used for recognition are free of spurious resolution or phase reversals". Because recognition requires both sufficient spatial frequencies and above-threshold contrast, this suggests that the highest spatial frequencies required for letter recognition are limited by the threshold contrast and do not exceed the first "zero crossing" of the MTF of the observer's retinal image. Since this conclusion is crucial to the development of any model predictive of clinical measures of human visual performance [Lang, 1993], we tested it using a method which allows concurrent assessment of letter images and measurement of the MTF of an optical model of the human eye which generates the images. As seen below, this procedure allows correlation of letter recognition with the location of spurious modulation. It does not measure contrast levels for threshold recognition.