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.
Plain English
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.
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 surface. Knowing this helps explain why depth perception depends on the eye, not just the object.
Why Pilots Care
Accurate interpretation prevents misjudging runway distance or aircraft separation during approach and landing.
Analogy
It is like looking at a person walking away from you. The person has not become smaller, but the picture of that person in your eye takes up less space, so you sense that they are farther away.
Grounding Statement
As an object moves farther away, the picture it forms in your eye gets smaller even though the object itself has not changed size.
Intuition Check
Retinal image size does not mean the real size of the object. It means how large the object appears on the seeing surface inside your eye.
Example Sentence 1
Because the runway was much wider than he was used to, the small retinal image of the runway edges made the pilot believe he was higher than he actually was on final approach.
Example Sentence 2
A distant aircraft showed a small retinal image size, confirming it posed no immediate conflict.