Take a second and try this, close an eye and then stick out your index fingers and point them at each other. Start about six inches apart and bring them together quickly until the touch. Did you line up perfectly? Chances are you missed by a little. Now try it with both eyes open.
Go ahead, I will wait
It is much easier to have your two fingers align squarely with both eye open.This is because you have binocular vision; it provides us the ability to discriminate small changes in distance when using two eyes. It goes away when we close an eye. It works using the idea of “normal” double vision.
Now I want you to look out your window off in the distance, there is probably a car and a building. Which one is closer too you? Now close an eye, is it more difficult to tell?
It shouldn’t be.
At distances greater than arms length we really do not use binocular vision. We can judge depth with one eye or both eyes equally.
Depth perception means the ability to determine what is closer to us, but the tools we use to do this vary. Up close the most important one is binocular vision. At distance binocular vision really is not useful, we use other tools there including shadowing, lighting, and obstruction (meaning if a car blocks our view of a house, we know the car is closer).
Binocular vision requires two well aligned, well seeing eyes. Therefore individuals with eyeturns (strabismus) or large amount of amblyopia in one eye will not develop binocular vision (without treatment). However these individuals will still have depth perception but up close they may find it more difficult to thread a needle or cap a pen.The other common drawback is they will not appreciate 3-D movies such as Avatar or Alice in Wonderland.
To sum up, if you lack binocular vision it does not mean you don’t have depth perception. You can still judge depth, but judging small distances up close will be more difficult.
William Dodge Perry, OD
Optometrist, Falls Church VA