# How does viewing through two eyes give us a sense of depth?

With ONE ray, your eye can never determine where the object is. Note that in your diagram, the eye can deduce the line along which the apparent image is, but to make a point, we need two lines! Who told the eye that the apparent object was directly above the real object? So there has to be a second ray of light. - Manishearth

I sort of get the final experimental result, but I don't understand 'how' or what exactly is going on here. Could it be explained in more details how precisely it is that viewing through two eyes give us an illusion of depth?

One of main confusion comes from the fact that I think that depth comes from prespective. For instance, have a look at this photo:

In the above photo whether I open only one or two eye, it seems so that that the feeling of depth is preserved. I suppose that it would still be if I was standing in the physical place where the picture is saying.

• Related question Commented May 30, 2022 at 21:41
• You can’t get parallax by looking at a flat photograph with two eyes. Two search terms are “stereoscope” and “anamorphic art.” You might also go to a 3D movie and experiment with taking the special glasses off, or holding them funny and sending the “wrong” image to each eye.
– rob
Commented May 30, 2022 at 21:56
• Damn this anamorphic art is mind blowing and cool.. but I am not sure how this helps me understand the question more @rob Commented May 30, 2022 at 22:03
• You might also try to visit an Ames Room. The effect only works with one eye. Lots of anamorphic art feels like “oh, okay” when you see it in person with two eyes, no matter how impressive the photos and videos.
– rob
Commented May 30, 2022 at 22:21
• That hurt my brain.. very cool tho. @rob Commented May 30, 2022 at 22:33

The photo is, of course, flat. So your feeling of depth when gazing upon it comes from your experience in the 3D world. Indeed, when looking at a scene, say, 1 km away, the distance to your face is much larger than the separation of your eyes. Your eyes see essentially the same thing. Therefore, much of your experience of visual depth in the world comes from interpretation of a 2D image (which your brain is highly evolved to do). You have learned that farther things appear smaller, much as you’ve learned that, e.g., dropped things fall. Your brain uses this information without you being aware of its existence.

Stereoscopic vision is important for viewing scenes at a distance on the length scale of your body. In this case, the spread between your eyes significant, and your brain interprets the two slightly different images as depth. First, try looking at an object a couple feet in front of you and alternately closing your left then your right eyes. This will give you a sense of the information your brain receives. Next, try closing one eye and trying to pick up small objects an arm’s length away. Is it more difficult to gauge depth than with two eyes?

• I was looking for a quantitative answer... something with numbers. I sort of get the qualitative intuition Commented May 30, 2022 at 23:44
• @Aplateofmomos At the New River Gorge Bridge in West Virginia, the ridge-to-ridge distance across the gorge is about half again the distance for which most people have parallax-based 3D vision. So if you stand at the overlook and look across the river, you have a sensation of depth for the trees at the water’s edge. But about halfway up the far side, your depth perception fails, and the trees above/beyond that point look “flat.” There is kind of a “last 3D row” of trees, and most people at the overlook agree where it is. It’s an interesting sensation.
– rob
Commented May 31, 2022 at 3:30
• The bridge is about 900m long. I haven’t been on that overlook in many years, but I would ballpark the depth-disappearing effect at 600–700 meters, substantially farther than the “100 feet” estimate in v1 of this answer.
– rob
Commented May 31, 2022 at 3:44
• @Aplateofmomos for the mathematics sites.google.com/site/howspaceisperceived/home/the-mathematics Commented May 31, 2022 at 3:51
• @rob indeed, 100 ft is too short. I just verified this by observing the parallax between a tree ~300 ft away against a much farther background when moving my head the distance of my eye separation side to side. Parallax was clearly visible at that distance. I increased the answer’s value to 1 km. Commented May 31, 2022 at 18:01