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Defocus blur is a useful cue in many natural tasks. Although blur discrimination has been studied extensively, defocus sensitivity in natural scenes has not been systematically investigated. Here, we use a collection of natural image patches, sampled from well-focused photographs, to measure discrimination of blur created by the eye’s optics. We constructed a psychophysical rig capable of presenting stimuli at three physical distances simultaneously along the line of sight. Half-silvered mirrors combined the 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. A calibration procedure eliminated spurious geometrical, color, and luminance cues. Subjects viewed stimuli monocularly through a 4 mm artificial pupil. Accommodation was not paralyzed. At the …