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New Zealand Stereoscopic Society Article.
3-D Camera Base Separation by Marc Dawson.

How far apart to have Left and Right cameras depends on the width of your intended screen display and how close you are to the scene.

Camera base separation of 1/30th the distance from the nearest object is common advice. This stems from purpose built 3-D film cameras having 65mm lens separation, 1/30th of the 2 metre wide screen used for polarised slide display. Film camera views overlap at approximately 2 meters wide, 2 meters away. So lens separation of 1/30th the distance from the nearest object applies for intended screen display of up to 2 metres wide.
With two separate cameras, you have an opportunity to adjust the lens base separation to suit the scene. Unlike 2-D screen image, 3-D images have a built in maximum screen size that is limited by the disparity of the views. In an anaglyph image the disparity is demonstrated with the color fringes around objects. When the disparity is wider than your eye spacing, it becomes difficult if not impossible to view.
Consider that you intend to display 3-D on a 2 meter wide screen that will be ideally viewed from 4 meters away.
The most realistic you can make the 3-D image appear is by filming with the cameras separated at your eye spacing, 62.5mm, converging the cameras at 4 meters away, by zoom or lens angle, to 2 meters wide at that distance. This creates an orthoscopic 3-D screen image with realistic image size and parallax 1:1 in scale viewed from 4 meters away on a 2 meter wide screen. Infinity, the very distant horizon, will have your eyes viewing parallel as in natural viewing.
The image as described above will also look fine on screens smaller than 2 meters. But it will no longer be orthostereoscopic. Footage intended for small screens need not maintain a parallel eyes view of infinity in order to avoid the image disparity from overwhealming the small screen width.
But if the above image is displayed on a screen twice as wide, the disparity also doubles and your eyes will then have to diverge wider than parallel to see the the image background. The closer you are to the screen the more your eyes will be required to diverge. Viewing from further back will help narrow the viewing angle. You may get away with a little bit of excess separation, but not double, especially for novice viewers. To assist, when projecting the image, converging the projectors will reduce the background disparity, but at the expense of having the foreground float in front of the screen. The floating screen effect is usefull for cinema size screen projection.
If you zoom in on 1 Meter wide views of your scene, ie half your intended orthoscopic screen size from 4 Meters back, the image size will be doubled on your 2 meter screen. So will the disparity, making the image unviewable. So reduce the lens separation to half of your eye spacing to maintain an orthoscopic view. If you cannot get the cameras closer together in order to half their separation, then zoom in from 8 Meters back in order to half the parallax to maintain viewability. The zoom will flatten the stereo perspective and so the image will not be orthostereoscopic, but it will be viewable with the same parallel eyes view for infinity.
If you zoom out to 3 Metre wide views, 1.5 times wider than your intended screen, you may then make the camera separation up to 1.5 times your eye spacing. Or it will look fine left as is.
With reference to the intended screen width and ideal viewing distance, reduce the lense separation proportionaly when filming from closer and increase lense separation proportionaly when filming from further back. If you film from 2 Meters away with wide angles converged at 2 Meters wide, it will cause the ideall screen viewing distance to be 2 Meters from the screen. The view from further back will look fine but will appear stretched, accentuated, and not quite as realistic. So reduce lense separation by half for your viewers sitting at four meters.
Keeping well within these guidelines will help ensure a consistent viewability and perception of image depth. However, filming up to this guideline will sooner or later lead to unviewable footage for your screen size. So aim to undershoot the camera separation especially when you have a large screen size in mind.
If the camera base is too large it will not help the 3-D effect, it will reduce the potential screen display size.

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