The utility of defocus blur in binocular depth perception

The question of whether defocus blur is a quantitative cue for depth perception is a topic of renewed interest. A recent study suggests that relative defocus blur can be used in computing depth throughout the visual field, particularly in regions where disparity loses precision. However, elements of the study's experimental design and theoretical analysis appear to undermine this claim. First, the study did not provide evidence that blur can be used as a quantitative depth cue. It only measured blur discrimination thresholds, not perceived depth from for blur. Second, the study's conceptualization of the complementary use of blur and disparity, and related conjectures, are based on the specific viewing geometry and fixation distance tested. They do not appear to generalize to natural viewing situations and tasks. I suggest a different way in which defocus blur might affect depth perception. Because depth-of-focus blur is a cue to egocentric distance, it could contribute to quantitative depth perception by scaling depth relations specified by other relative depth cues.