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Each of our eyes sees a slightly different view of the physical world. Disparity is the small difference in position of features in the retinal images; stereopsis is the percept of depth from disparity. A distance between corresponding features in the retinal images of the two eyes smaller than the “upper disparity limit” yields a percept of depth; distances greater than this limit cause the two unfused monocular features to appear flattened into the fixation plane. This behavioral disparity limit is consistent with neurophysiological estimates of the largest disparity scale in primate, allowing us to relate physiological limits on plausible binocular interactions to separation between retinal locations. Here we test the hypothesis that this upper disparity limit predicts the presence of coarse stereopsis in humans with macular degeneration (MD), which affects the central retina but typically spares the periphery. The pattern of vision loss can be highly asymmetric, such that an intact location in one eye has a corresponding point in the other eye that lies within affected retina. Nevertheless, some individuals with MD have coarse stereopsis that is useful for eye-hand coordination. Our results show that individuals with MD (n = 25, male and female) have coarse stereopsis when the distance between intact retinal locations is less than the behavioral and physiological upper disparity limit at the corresponding eccentricity. Furthermore, for those without stereopsis, we can predict whether they can achieve stereopsis by using alternate retinal loci at further eccentricities whose separation is below the upper disparity limit. SIGNIFICANCE STATEMENT We show that the largest separation between features in the two eyes that yields a percept of depth in humans is related to the largest disparity scale in macaque medial temporal area and to the estimated size of the receptive fields in human depth-sensitive cortical regions. This upper disparity limit also predicts whether individuals with retinal damage due to macular degeneration will have stereopsis. Individuals have stereopsis when the separation between intact retinal locations in the two eyes is smaller than the upper disparity limit measured behaviorally. Our results indicate the importance of the behavioral upper disparity limit as a predictor for stereopsis in populations with retinal damage.

Purpose The PowerRef 3 is frequently used in studying the near triad of accommodation, vergence and pupil responses in normal and clinical populations. Within a range, the defocus measurement of the PowerRef 3 is linearly related to the eye's defocus. While the default factory‐calibrated slope of this relation (calibration factor) is 1, it has been shown that the slope can vary across individuals. Here, we addressed the impact of changes in viewing distance, age and defocus of the eye on the calibration factor. Methods We manipulated viewing distance (40 cm, 1 m and 6 m) and recruited participants with a range of accommodative capabilities: participants in their 20s, 40s and over 60 years old. To test whether any effect was larger than the range of measurement reliability of the instrument, we collected data for each condition four times: two in the same session, another on the same day, and one on a different day. Results The results demonstrated that viewing distance did not affect the calibration factor over the linear range, regardless of age or uncorrected refractive error. The largest proportion of the variance was explained by between‐subject differences. Conclusions Calibration data for the PowerRef 3 were not sensitive to changes in viewing distance. Nevertheless, our results re‐emphasise the relevance of calibration for studies of individual participants.