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Spatial Compression Produced by a Stationary Low-Vision Telescope

Purpose. Geometrical analysis of the monocular information for visual space perception predicts that the magnification produced by a low-vision telescope will compress the depth dimension of space. To test this prediction we measured the compression in depth of perceived shape while looking through a stationary telescope. To control for the other aspects of telescopic viewing, apart from magnification, we also measured perception while looking through a plain tube having the same field of view. Methods. A 2.75× Keplarian telescope was mounted 40 cm above a tabletop patterned with receding stripes. The 11.6° field of view was centered on a series of rectangular stimulus cards lying flat on the table at a distance of 100 cm. Participants monocularly viewed each card through the telescope, or through a tube having the same field of view, and verbally judged the card’s perceived length (in depth) relative to its width (in the frontal plane). Results. Perceptual compression of shape was calculated by dividing the perceived proportion (length/width) by the actual proportion. The telescope and the tube both produced significant perceptual compression, but perception was significantly more compressed through the telescope (0.43) than through the tube (0.52). Conclusions. The magnification produced by a stationary low-vision telescope can result in a compression of perceived depth. In addition, other aspects of telescopic viewing, such as monocular vision, restricted head movements, and a restricted field of view, can together contribute substantially to such compression. Further research is needed to assess the clinical implications of these results.