Depth Perception, What Exactly is It?
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 by 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. Take a look at the picture of the baseball below:
Now hold up one finger about six inches away from your face, in front of the picture of the baseball. If you focus your eyes on the baseball, you will see two somewhat transparent images of your finger, on either side of the baseball.
Now focus your eyes on your finger. You will see two somewhat transparent images of the baseball, 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 baseball, just like before. Focus on the baseball and close your left eye. Your finger should appear to the left of the baseball. Now open your left eye and close your right eye. Your finger will seem to jump to the right of the baseball.
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 baseball, 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 don’t 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 don’t 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, perhaps 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!
Another way either one or both eyes can perceive depth is by perspective – which means how objects appear based on the position of your eyes in relation to the object.
Here’s a very common way we perceive perspective. When we’re standing on the ground and looking straight down at parallel lines, they appear parallel:
If you click the below link it will take you another tab. Click the red “add perspective” ball, and you’ll see the lines converge (and be sure to come back when you are done). It’s like when you look at railroad tracks off in the distance as they approach the horizon. They appear to meet at what is called the “vanishing point”:
Just like the way perspective can trick the eye into seeing parallel lines that appear impossibly to meet, artists can use perspective tricks to create other optical illusions. Look at this one, created by the German psychiatrist Franz Muller-Lyer in 1889:
The two parallel lines are the exact same length. You can prove it with your ruler. But the line on the top appears longer, because our brains perceive the inverted angles as lengthening the line.
What causes this? Theorists have advanced various explanations, none proven. If you are interested in exploring this further, you can read the explanations at the link below, and test their validity yourself:
Your perspective affects not just your view of distance, but the appearance of colors and hues you perceive as well. Check out this picture:
The gray bar across the middle of the picture appears lighter on the left and darker on the right. But that’s only because of the contrast between the gray bar and the different shades of gray in the background. The background is darker on the left and lighter on the right. Despite its appearance, the gray bar is actually the same color on the right and left.
In addition to appearing to vary shades of a color, sometimes our visual perspective can cause us to see colors that are not actually there. This is because our brains can perceive color in relation to other colors that are present. Take a look at this optical illusion, follow the simple instructions, and notice if you see green grass:
These optical illusions are all results of perspective – what you perceive as true is determined by your perspective, a principle that can apply not only in vision but in many other aspects of life, as well!
As you ponder this, here are some other fun optical illusions to look at and share with your family and friends, to get their perspective, too!