Size constancy

Size constancy is the tendency for any given object to appear the same size whether its size in the retinal image is large or small. For example, if you see someone walking towards you, their retinal image increases progressively, but their size seems to remain the same. Reasonable or high levels of size constancy have been obtained in numerous studies (see Goldstein, 1996).

Most research on size constancy has been carried out in the laboratory, and Brunswik (1956) argued that it was important to study size constancy in the external environment. Accordingly, he asked a student to walk around outdoors and to provide estimates of the sizes of numerous objects. She did this very successfully, and there was a correlation of +.99 between object size and judged size. This high level of performance did not depend simply on the use of information about retinal size. Brunswik found that the correlation between actual object size and retinal image size was +.7 across all objects, but it fell to only +.1 when small objects were excluded from the analysis.

There is a potential problem with Brunswik's study. It is not clear whether the student's estimated sizes of objects reflected what she saw or what she knew to be the case. For example, a large oak tree a long way from us may look fairly small, even though we know that it is probably rather large.

Why do we show size constancy? A key reason is that we take account of an object's apparent distance when judging its size. For example, an object may be judged to be large even though its retinal image is very small if it is a long way away. The fact that size constancy is often not shown when we look at objects on the ground from the top of a tall building or from a plane may occur because it is hard to judge distance accurately These ideas are incorporated into the size-distance invariance hypothesis (Kilpatrick & Ittelson, 1953), according to which for a given size of retinal image, the perceived size of an object is proportional to its perceived distance.

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