Can random-dot stereograms serve as a model for the perception of depth in relation to real three-dimensional objects?

The ability to perceive depth in a random-dot stereogram is a valuable test for the perception of retinal image disparities, whether they arise from the viewing of a stereogram or from the viewing of a real 3-D object. However, a stereogram cannot be regarded as a proper model for the perception of depth in the case of a real 3-D object. This conclusion comes out most clearly in relation to changes in viewing distance. Whereas the viewing of real objects and stereograms both obey the rules of size constancy, this is not the case with depth constancy. With changes in viewing distance, the viewing of real objects obeys the rules of depth constancy. By contrast, the magnitude of the depth intervals in a stereogram are not constant but appear to increase in direct proportion to the increase in viewing distance. In a stereogram these changes in the amplitude of the depth intervals are based on the same mechanisms as those responsible for size constancy.