To allow for the perception of size, we have a mechanism that balances the perspective distortion of objects. It guarantees that the changing trapezoidal and ellipsoidal forms in the retina image can be perceived spatially as being normal, rectangular or round objects by being aware of the angle at which the object is viewed.

Constancy with regard to perception of size. Due to the perspective interpretation of this illustration, the luminaires are all perceived as being the same size in spite of the variations in size of the retina images.