Abstract
Accommodation and vergence are considered to be the main source of signals to absolute depth, since retinal visual cues are thought to signal only relative depth relations. Focal blur in the retinal image has been shown to be a reliable signal to unsigned ordinal depth relations. However, focal blur does contain information that signals absolute distance, since the rate of blurring around the fixated point varies systematically with change in absolute depth of fixation for a given pupil diameter. For example, the gradient of blur around a point of fixation between 10 and 30 cm is usually much higher than that for fixation beyond one meter. At distances much greater than a meter, the gradient of blur is significantly diminished. The gradient of blur should typically be greatest along the vertical meridian since the lower hemi field usually contains objects closer than fixation, while the upper hemi field typically contains objects further away. A curious visual effect, that has been recently exploited by photographers, makes objects in photographed scenes with a high degree of focal blur appear much smaller than reality. This suggests an effect on size constancy due to a change in perceived distance. Observers compared scenes (containing either circular patches or spheres) presented on two separate displays which had different degrees of simulated focal blur. The displays could be independently set at different distances from the observer. Observers matched the perceived size or distance of either a target patch or sphere in the displayed scenes. Settings showed a small effect of blur on perceived distance and size, where an object surrounded by higher blur appeared closer and smaller than one surrounded by elements that were in focus. We consider how a small but reliable signal to absolute depth could result in the vivid effects seen in pictorial images.