Have fun with strabismus!

Experience 3D by 2D methods (Part 2)

This is how the atmosphere influences contrast and color of distant objects; for example a skyline in the fog (or smog). This effect is often used in art and photography (and computer games) to enhance the depth effect of images.

Accommodation

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.

Experience

The 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.
This combination of a commercial advertisement and someone waiting for the train might look as one strangely proportioned person. Experience has taught it is likely that someone is standing behind the picture, however. By the way, for once, this picture isn't an electronic fake.

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.

Convergence

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.

Many tests used for the evaluation of stereopsis are not well suited for those who have limited fusional abilities; for example red/green glasses combined with pictures with much detail, and random dot stereograms, provide such strongly different inputs that it becomes very hard to find similar points on which to base fusion. In daily life you may very well have stereopsis (and use it too) and still a test could lead to the (incorrect) conclusion that you have none. The more the test resembles everyday viewing conditions, the more accurate it will be.