Human visual system

Andrew Dalke dalke at
Mon Oct 2 03:12:45 EST 2000

Mark Defaria asked:
>I had a question.  I was wondering why is itwhen an object is closer to us
>that we can see it in more detail and what aspect of the human visual
>system is responsible for this?

Lenses, including eyes, don't actually see size directly but see angular
width.  The farther away an object is, the smaller the angle it subtends.
The reason we think of "size" rather than "angular width" is because
people have stereoscopic vision, which gives the distance component of
the equation.

There is a minimum angular width we can see, which related to the
size of the light receptor on the retina.  Everything in that solid
angle gets mixed into a single point.

So when something is far enough away, its angular width is very small
and only picked up by a single receptor in the eye.  It doesn't matter
how the object really is - it's perceived as a dot with no other detail.

As it comes closer, the angular width increases, and neighboring receptors
in the retina are exposed to different parts of the object.  So you
see more detail.

                    dalke at

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