Point of convergence (3D filmmaking)

by Rogers Jayzee

Hold a pencil 1 foot in front of your face and look at it. You will feel your eyes crossing to look at it. Now the pencil is the point of convergence, POC.

While looking at the pencil, anything further away than it will be doubled (diverged). You will see 2 of them. The POC is converged, everything else is diverged. This is how we see depth (binocular vision).

Think of a line drawn from one eye to the other. Now think of a line drawn from each eye to the pencil. These 3 lines form a triangle. The point of the triangle is the POC. It is where both eyes converge on the same object.

Your eyes NEVER look straight ahead. When you are looking at anything, no matter how far away, your eyes still converge on some part of that image. Your eyes and THAT part of the object form a triangle.

Have you ever noticed that when you look into someone's eyes, you can only look into one eye at a time? It's because your eyes have to converge on something. We don't just gaze. We converge or reconcile for the visual disparity between the left and right image.

Our eyes bring the two images together in our brain to form a single image with depth. So what does that have to do with 3D images/video?

Well, when you look at the screen in the theater, your eyes are converging upon the screen. That is the POC.

When you look at a 3D movie, the images at the POC are already converged (the left and right images) so, your eyes are able to simply converge on the screen. That is why the images at the POC appear to be on the screen.

Now, objects that were photographed further away than the POC will not be converged. They will appear as 2 offset images (one for the left eye, one for the right). When you watch the movie, your eyes will have to adjust their POC to bring the 2 offset images together just the way they would have to if they were looking at the scene live.

Also, when one part of the image is converged by your eyes, another part will then diverge, just like actual sight (binocular vision). Thus, we get a sense of depth.

By the way, it works the same for object that were photographed closer than the POC. When you watch the movie, your eyes will have to cross to converge the close images. Everthing ele will diverge as expected and your brain will get the sensation of viewing something coming out of the screen.