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 eyea??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 subjecta??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 beginning of each trial, subjects judged the orientation of a low-contrast, high-frequency grating on the focus monitor. This ensured accurate accommodation at the focus distance of 80 cm. Next, two cosine-windowed natural patches were presented on the stimulus monitors for 150 ms. The task was to identify the sharper patch in a 2AFC paradigm according to method of constant stimuli (standard from 0.0 to 0.75 D). Discrimination thresholds varied substantially between stimuli but were correlated between subjects. Consistent with the literature, thresholds decreased with defocus pedestal. For comparison, the Maltese cross stimulus, the most widely used artificial stimulus in defocus research, yielded poorer performance than ~80% of natural stimuli, suggesting that the usefulness of defocus cues has been underappreciated. The lowest thresholds were at or below the lowest thresholds ever reported.
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