*
by
Rogerz Jayzee
*

There is a lot to consider when setting up a stereo image to be photographed. I will go over the basics now but they are only the basics.

First, stereo (or 3D) images do not have infinite depth. You have to work within the “3D window” . That is, the nearest and farthest point you can shoot that will produce a 3D image that does not put undue strain on the eyes of the viewer.

This window is mathematically arrived at and takes into consideration two variables; Point of Convergence (or zero parallax), and Interocular distance or Stereo base(distance between cameras).

These variables along with the maximum allowable divergence of the left and right images (how cross eyed the pictures can be) are used to calculate the near and far distance that is allowed.

The 3D window has 3 places for things in the shot to be located. Things are either on the screen, in front of the screen or behind the screen.

It is very important to know where you want things to be before you photograph them. You need to know how far from the camera the objects that will appear ON THE SCREEN will be. This is called Point of Convergence (POC) or zero parallax.

Once you know this, and you know how far away your cameras will be separated(stereo base), you can mathematically come up with the near and far distances.

The near distance is ‘how far in front of POC something can be in the shot’. The far distance is ‘how far behind the POC something can be in the shot’. These numbers are expressed as ‘distance from the camera’, not ‘distance from the POC’.

The importance of the interocular distance or stereo base (distance between the cameras) has to do with how much 3D information will be gathered.

Basically, the closer the cameras are to each other, the greater the near and far distances will be BUT, the more distant objects will appear flat.

The further apart the cameras are from each other, the less distance between the near and far distance BUT, the more round or 3D in appearance objects will be.

If you used a 2.5 inch interocular distance and wanted to shoot a bunch of trees in the far distance, each tree will end up looking like a cardboard cutout. You would still be able to see the distance between the trees but the trees themselves would not look round or 3D.

For that kind of a shot you might want to separate the cameras by as much as 1/20th or 1/30th of the distance from the trees to the camera to make a really good 3D picture. Depending on how far the trees are away from the camera, that distance could be 6 inches to 100 feet or more.

There is a lot to consider and these paragraphs do not begin to explain the complexities of it.

Here is a list of values that I have calculated for you to show some 3D solutions. I have built a stereo calculator that automatically gives me these numbers.

So, if the stereo base is 2.5 inches and the POC is 12 feet then the closes object in the frame can be no closer than 7 feet from the camera and the furthest object in the frame can be no more than 42.5 feet from the camera. That is the 3D window.

If the stereo base is changed to 4 inches and the POC is left at 12 feet then the near distance changes to 8.5 feet from the camera and the far distance changes to 21.5 feet from the camera.

As you can see, the wider stereo base results in much more shallow 3D window.

Here are a pair of left and right images that I took:

These pictures were taken with a single camera on a tripod. The stereo base is 6 inches. The POC is 35 feet. The near distance is 19 feet and the far distance is 279.5 feet.

As you can tell from the pictures, the distant mountains are much further away than 279.5 feet however, mathematically speaking, the greater the far distance is from the camera, the less change in divergence occurs as you move even farther away.

In other words the difference in divergence between that of 279.5 feet and the mountains (infinity) is so small that it can be considered an allowable error. You would not want this kind of an error for an entire movie but you can have them for short periods of time. The larger the error, the less time you want to subject your audience to them or they may experience eye fatigue and possibly headaches.

This kind of consideration is called 3D budgeting. If you budget the time people are exposed to certain degrees of stereo divergence, you can keep the experience from becoming unpleasant.

**
**