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Defocus blur is a useful cue in many biological tasks. However, the extent to which the variability in natural images affects the discriminability of defocus blur is unknown. Here, we measured discrimination of retinal image blur created by the eye's optics, for a large collection of well-focused natural image patches. We constructed a psychophysical apparatus capable of presenting stimuli at three distances simultaneously along the line of sight. Half-silvered mirrors combined light from three monitors positioned at variable distances from the subject. The stimuli were rendered sharply on each monitor so that defocus blur was created by the optics of the subject's eye, as in natural viewing. Observers viewed stimuli through a 4 mm artificial pupil. Accommodation was paralyzed. On each trial, two different natural image patches were foveally presented in a 2IFC paradigm. The observer identified the interval with the …