Plain English Meaning

How big something looks to your eye. Things that are closer look bigger, and things that are farther away look smaller. Your brain uses that apparent size to judge distance.

Full Definition

The size of the image that an object projects onto the retina at the back of the eye. The brain uses this projected size, together with the known or assumed actual size of the object, as one of the visual cues for estimating how far away the object is. A larger retinal image suggests the object is closer; a smaller retinal image suggests it is farther away.

Context Anchor

Seen in discussions of distance estimation and depth perception, especially when judging runway distance, traffic distance, and approach perspective by sight.

Derivation

Retinal comes from retina, the light-sensing layer at the back of the eye, from Latin rete meaning net (the retina has a net-like pattern of blood vessels). Image size simply means how large the picture of the object is on that . Knowing this helps why depth perception depends on the eye, not just the object.