Equiblur zones at the fovea and near retinal periphery

Knowledge regarding successive blur discrimination thresholds (i.e., equiblur zones) in depth and across the near retinal periphery, and their relation to blur detection (i.e., depth-of-focus), remains unknown. The blur detection threshold and four successive blur discrimination thresholds were measured psychophysically at the fovea, as well as at retinal eccentricities of 0.25°, 2°, 4°, and 8°. A Badal optometer system was used to assess blur sensitivity monocularly in five visually normal young adults with cycloplegia. The foveal test stimulus consisted of a small irregularly shaped black form, and the peripheral test stimulus consisted of high contrast circular apertures of different radii. Both the group mean blur detection and successive blur discrimination thresholds progressively increased with retinal eccentricity. At retinal eccentricities of 0°, 0.25°, 2°, 4°, and 8°, the group mean blur detection thresholds were 0.53 ± 0.06 D, 0.59 ± 0.10 D, 0.93 ± 0.11 D, 0.98 ± 0.16 D, and 1.25 ± 0.25 D, while the average values of the group mean blur discrimination thresholds across the steps were 0.29 ± 0.01 D, 0.37 ± 0.01 D, 0.48 ± 0.00 D, 0.51 ± 0.01 D, and 0.72 ± 0.02 D, respectively. At each retinal eccentricity, the blur discrimination thresholds were similar to each other, and they were approximately 60% of the blur detection threshold magnitude. These findings provide a conceptual representation of blur perception throughout the central visual field. Possible mechanisms are proposed for the decreased blur sensitivity in the near retinal periphery, as well as for the difference between the blur detection and blur discrimination thresholds.