Have you seen the movie Gravity ? Did you see it in 3-D?

If so, you probably found yourself marveling at the amazing technological effect of seeing Sandra Bullock and George Clooney tumble through a very realistic depiction of the depths of outer space.

But you probably never find yourself marveling at the even more amazing technological effect of seeing everything in the world in 3-D – achieved by simply having two eyes, as well as a brain that blends the images your eyes see.

Very simply, this is depth perception, meaning the ability to see things in three dimensions – length, width, and depth – and to perceive how near or far away people and objects are.

One way humans perceive depth is having eyes that are spaced a little bit apart. This is contrary to certain animals that tend to be prey, and have evolved so that their eyes are on either side of their head to enable them to spot predators. (Predator animals, on the other hand, or paw, usually have eyes positioned closely together, like humans, to enable them to spot, chase, and swoop down upon prey.)

Like those clunky 3-D glasses you wore while watching Gravity , human eyes spaced a little bit apart see two slightly different images, based on each eye's position on your face. These two slightly different images are called binocular visual cues. They're called binocular because the images are seen by two eyes.

Here's a fun way to test your binocular vision. Hold your cell phone at arm's length in front of your face.

Now hold up one finger about six inches away from your face, in front of the cell phone. If you focus your eyes on the cell phone, you will see two different transparent images of your finger, on either side of the cell phone.

Now focus your eyes on your finger. You will see two different transparent images of the cell phone, on either side of your finger. That's depth perception.

Here's another way to test your depth perception, and to see whether one of your eyes is dominant.

Hold your finger in front of the cell phone, just like before. Focus on the cell phone and close your left eye. Your finger should appear to the left of the cell phone. Now open your left eye and close your right eye. Your finger will seem to jump to the right of the cell phone.

Once again, as in the previous test, if this is what you see, you have binocular – also known as stereo – vision. However, you may have faulty depth perception if you experience any of these things:

– You can see your finger better on one side than the other.

– The view of your finger is larger with one eye than with the other.

– One eye's finger image appears right over the cell phone, while the other one is far to the left or right.

– You can only see one finger image.

If you do experience any of these things, it would be a good idea to get your eyes checked by an ophthalmologist or an optometrist.

But what if you have sight in only one eye? Do you still have depth perception, even if you do not have binocular (stereo) vision?

Yes! Here is a way to test your one good eye for depth perception.

Let's say you are doing this indoors. You will need stationary objects in three different distances from you: one object close to you, one a little farther away, and one still farther away from that one.

Let's try this with three objects you may find indoors: a chair (close to you), a filing cabinet (a little farther away), and a table (farther away).

You can do this outside, too. Try it with, say, a bush (close to you), a tree (a little farther away), and a house (farther away).

Of course you do not have to use these three indoor and outdoor objects, in these three positions. You can use them in different positions, or you can use three other objects, as long as the one farthest away is large enough so you can see it well.

Focus on the object in the mid-distance range – the table if you are inside, the tree if you are outside, if you are using those objects. Then walk from side to side.

As you focus on the midrange object, sometimes the filing cabinet or tree, you will see the object closer to you (the chair or the bush) move to the opposite direction from where you are walking. You will also see the object farther away (the table or the house) appear to move in the same direction that you are moving.

If you can see this with one eye (if you are sighted in both eyes you can close one eye while you do this), you have depth perception!