This makes intuitive sense: an object of constant size will project progressively smaller retinal images as its distance from the observer increases.
[4] Under that interpretation, Emmert's law is a special instance of size constancy and of the size–distance invariance hypothesis, which states that the ratio of perceived linear size to perceived distance is a simple function of the visual angle.
However, the increase in perceived size is much less than would be predicted by geometry, which casts some doubt on the geometrical interpretation given above.
[7][8] A neuroimaging study that examined brain activation when participants viewed afterimages on surfaces placed at different distances found evidence supporting Emmert's Law and thus size constancy played out in primary visual cortex (V1); i.e. the larger the perceived size of the afterimage, the larger the retinotopic activation in V1.
[9] Some have criticized the use of Emmert's law as an explanation for phenomena such as the moon illusion, because Emmert's law explains one perception in terms of another, rather than explaining any of the complex internal processes or mechanisms presumably involved in perception.