Spatial compression produced by a stationary low-vision telescope

Direct manipulation of perceived angular declination affects perceived size and distance: a replication and extension of Wallach and O'Leary (1982)

In two experiments involving a total of 83 participants, the effect of vertical angular optical compression on the perceived distance and size of a target on the ground was investigated. Replicating an earlier report (Wallach & O’Leary, 1982), reducing the apparent angular declination below the horizon produced apparent object width increases (by 33 %), consistent with the perception of a greater ground distance to the object. A throwing task confirmed that perceived distance was indeed altered by about 33 %. The results are discussed in relation to cue recruitment and to recent evidence of systematic bias in the perception of angular declination.

Compression of perceived depth as a function of viewing conditions

PURPOSE

The magnification produced by a low-vision telescope has been shown to compress perceived depth. Looking through such a telescope, however, also entails monocular viewing and visual field restriction, and these viewing conditions, taken together, were also shown to compress perceived depth. The research presented here quantitatively explores the separate effects of each of these viewing conditions on perceived depth.

METHODS

Participants made verbal estimates of the length, relative to the width, of rectangles presented in a controlled table-top setting. In experiment 1, the rectangles were either in the frontal plane or receding in depth, and they were viewed either binocularly or monocularly with an unrestricted field of view (FOV). In experiment 2, the rectangles were in depth and were viewed monocularly with an unrestricted FOV, a moderately (40 degrees) restricted FOV, or a severely (11.5 degrees) restricted FOV.

RESULTS

Viewed in the frontal plane, either monocularly or binocularly, the vertical dimension was expanded by about 10%. Viewed in depth, with an unrestricted FOV, the (projectively vertical) depth dimension was compressed by 12% when seen binocularly or 24% when seen monocularly. A monocular moderately (40 degrees) restricted FOV was very similar to the unrestricted monocular FOV. A severely (11.5 degrees) restricted FOV, however, produced a substantially greater 44% compression of perceived depth.

CONCLUSIONS

Even under near-optimal binocular viewing conditions, there is some compression of perceived depth. The compression found when viewing through a low-vision telescope has been shown to be substantially greater. In addition to the previously demonstrated contribution of telescopic magnification to this effect, we have now shown that the viewing conditions of monocularity and severely restricted (11.5 degrees) FOV can each produce substantial increments in the compression of perceived depth. We found, however, that a moderately restricted (40 degrees) FOV does not increase the compression found with unrestricted monocular viewing.