Two Eyes, One In-depth Picture
Is catching, juggling or heading a ball hard for you? If you ever tried threading a needle, did it end in frustration? Have you ever thought of blaming your eyes for this hardship? Two eyes that work together help you estimate how far a ball is, or where the thread is with respect to the needle. This “working together” of the eyes actually happens in the brain. The brain receives two images (one for each eye), processes them together with the other information received and returns one image, resulting in what we “see.” Are you curious about how depth perception enters the picture? “See” for yourself in this activity!
Humans have two eyes, yet we only see one image. We use our eyes in synergy (cooperating together) to gather information about our surroundings. Binocular (or two-eyed) vision has several advantages, one of which is the ability to see the world in three dimensions (also known as 3D). We can see depth and distance because our eyes are located at two different points (about three inches apart) on our head. Each eye looks at an item from a slightly different angle and registers a slightly different image on its retina (the back of the eye). The two images are sent to the brain, where the information gets processed. In a fraction of a second, our brain brings one three-dimensional image to our awareness. The three-dimensional aspect of the image allows us to perceive width, length, depth and distance between objects. Scientists refer to this as binocular stereopsis.
Artists use binocular stereopsis to create 3D films and 3D images. They show each eye a slightly different image. The two images show the objects as seen from slightly different angles, as would be when you saw the object in real life. For some people, it is easy to fuse two slightly different images presented at each eye. Other people find it hard. Their depth perception might rely more on other clues. They might find less pleasure in 3D pictures, movies or games, and certain tasks—like threading a needle or playing ball—might be more difficult for them.
- Three different-colored markers or highlighters that can easily stand vertically
- Place your markers, standing vertically, in a line. The first one 30 cm from the edge of the table. The next one 30 cm behind it (60 cm from the edge) and the last one 30 cm from the second marker (90 cm from the edge). If your table is not long enough, you can place your makers at 15 cm, 30 cm, and 45 cm from the edge of the table.
- Position yourself at the edge of the table, bend your knees so your eye-level is at the level of the markers.
- Close or cover your right eye and look only with the left eye. Shift your head so all three markers are right behind the other. Is it possible to hide the second and third marker behind the first one?
- Keep the position of your head the same, but now close or cover the left eye and look only with the right eye. What do you see? In your image, are the second and third marker still hidden behind the first one? Why do you think this happens?
- Reposition your head so the second and the third markers are hidden by the first one. Switch eyes with which you are looking again. Did it happen again? This time, observe some details. In your image, is the second marker to the right or the left of your first marker? What about the last marker? How far apart are the markers in your image? Do you see space between the first and the second marker? Do you see as much space between the second and the third marker (the markers that are further away from you)? Are some markers still partially overlapping?
- Open or uncover your right eye and look with both eyes. What do you see? Are any markers hidden by closer markers? Try to reposition your face so, in your image, the closer marker hides the more distant markers. Is it easy? Is it even possible?
- Use a camera to study this in more detail. Position the camera so the first marker hides the second and third marker. The tops of the markers can stick out. Take a picture.
- Shift your camera about 7.5 cm (3 inches) horizontally to the side, and take a second picture. Remember whether you shifted to the right or to the left. If you shift right, the first picture represents what the left eye sees. If you shift left, the first picture represents what your right eye sees.
- Look at the pictures. These images reflect what your right and left eye register (human eyes are about 7.5 cm apart). Are both pictures identical? In what way are they different?
- The brain uses the different location of objects in the images received by the right and the left eye to create a perception of depth. Can you find some rules the brain uses? Which marker you thing shifts most with respect to a distant point or with respect to the last marker, the closer marker or a farther away marker?
- In the first picture, the three markers are lined up. In the second they are not. Measure how much the second marker is shifted, with respect to the last marker. Now measure how much the first marker, which was positioned closer to the camera, is shifted with respect to the last marker. Does shift increase or decrease when objects are placed further away from the observer?
- Look at your second picture; is your second marker shifted to the right or the left with respect to the last marker? What about the first marker? Is this direction identical to the direction in which you shifted your camera?
- Can you imagine how the picture would look if you shifted the camera by about 7.5 cm to the other side? You can repeat part of the procedure where you take the pictures, but now shift your camera to the other side to find out.
Extra: Study other parameters that might influence the shift. Do the markers shift more or less with respect to each other if you (as observer) position yourself farther away from the set of markers? What happens if you gaze at a point far in the background (compare the shift with respect to a point in the background)? Pictures can help you perform a more detailed analysis. A row of equally spaced trees, light poles or other objects along a straight street can also help you perform a more elaborate investigation.
Extra: Imagine what would happen if our eyes were separated by a larger horizontal distance. Do you think the horizontal shift would be larger or smaller? What do you think would happen if our eyes were shifted vertically instead of horizontally? Take pictures where you position the camera at slightly different locations in space to find out. Can you find some advantages and disadvantages to having eyes that are separated as they currently are in humans?
Extra: Adequate depth perception facilitates tasks like playing ball, threading a needle and driving. To experience how difficult playing ball and threading a needle are with monocular (or one-eyed) vision, cover one eye and perform the task. Be careful, though; this is difficult! Start by throwing a ball softly. <i>Do not</i> drive any vehicle (including a bike) with one eye covered; you will not be able to adequately estimate the distance of obstacles or oncoming traffic.
Extra: If you have 3D pictures, can you find out what creates the perception of depth? Do you need to view them with a stereoscope, a virtual reality (VR) headset or red-blue glasses to get the depth perception? Why do you think this is the case?
Observations and Results
Did you see how your right eye registers the world differently from your left eye? Did you see how using both eyes created yet a different picture?
When you lined up the markers so your left eye could only see the first one, they were no longer lined up when you looked with the right eye only. Something similar happened when you lined up the markers for your right eye and you switched to a left-eye-only view. This time, the markers are shifted to the right in your image. This happens because each eye looks at the row of markers from a slightly different angle.
With both eyes open, it was probably very hard or impossible to position yourself so you only could see the first marker. Most people have a hard time fusing the images created by each eye in this particular setup. You might have experienced that you switched between images, or had double vision.
The pictures you took with the camera allowed you to compare how much a closer marker shifted with respect to a more-distant marker. If you performed some more tests, you might have discovered that the shift depends on the distance between the objects, the distance between you and the objects and the point you are gazing at (also called the point of focus).
Ask an Expert
- Perception Lecture Notes: Depth, Size and Shape, from Professor David Heeger, Department of Psychology, New York University.
- Starry Science: Measure Astronomical Distances Using Parallax, from Scientific American.
- Sight (Vision), from Neuroscience for Kids