3D the 2D way - Part 2
Experience 3D by 2D methods (Part 2)
As long as presbyopia hasn't set in, the crystalline lens in the eye accommodates to view things at different distances; its shape changes (becomes more or less convex); resembling the focusing mechanism of a camera. For some reptiles, this is all they need for very accurately detecting prey. In humans however, this mechanism is less accurate and not developed for this type of use. Here, the focus is on the leftmost pen, but it's impossible to say which one of the three is the nearest; in a picture, you can't accommodate on each one in turn, and compare. Blurring of objects in the distance is very often used in portrait photography; a crisp image of someone against a blurred, hazy background.
interpretation of these clues is based on past experience; on their own, without a huge
repository of known facts about the things we see, and knowlegde about the context in
which we perceive them, they would be useless.
As seen with two eyes
If the binocular vision has developed normally, you won't even be aware of it. If you focus on a faucet (arrow) it will not look like the one on the right, even if that is what is actually there to see.
For example if you
focus on a pencil held close to your face, everything in the distance will double up on
both sides of the pencil; but normally you won't notice it. It would be a confusing world
if you were! In theory, if one would have fusion but no stereopsis, the blurred double
images could work as a crude estimate of depth.
You would like a bigger car. Admit it. Here's how to do it:
Park it behind a lamppost (this might require some practice) and view the lamppost, not the car, from not too far away. The car appears to have gotten a little broader. Just that little extra edge one needs in today's competitive world.
When looking at something relatively close, the eyes turn in to converge on it. The angle of convergence indicates the distance, as in a range-finder. But this only indicates the distance of one object at a time. Possibly proprioception plays a role; some people can 'feel' the extracoular muscles pulling the eyes inward.
And finally, the added bonus of binocular vision
Stereopsis is binocular depth perception. It is based on the integration of the input
from the two eyes. Slight differences between the two retinal images are not perceived as
such, but fused into a single image, with extra information pertaining to the relative
location in space of the things you're looking at. It's not always true to say that if you
have strabimus, you have no stereopsis; stereopsis can come in many variations; some may
have very low-acuity stereopsis, others near normal, others none whatsoever, but even
amblyopic eyes can contribute to some form of stereopsis.