Question

What is meant by the observed size of an
object? How is it related to the angle subtended
by the object at the eye?​

Answer 1

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An object appears small or big depending on the size of its image on the retina of the eye. This apparent size of an object as perceived by the eye is called its observed size.

As an object is brought closer to the eye, accommodation permits the eye to increase its power and form a large retinal image. Also, the angle 8' subtended by the object at the eye when it is closer is greater than the angle 0 when it is more distant. This shows that the apparent size of an object depends on the angle subtended by the object at the eye.