3. Playback Rule
No person viewing a stereo image should be forced to diverge his eyes.
Why should the projection rule be adhered to?
When looking in the distance the viewing axes of the eyes are parallel, whereas looking at near objects the eyes converge inwards a bit. With natural vision, diverging viewing axes never occur.
Playback Rule: Avoid divergent viewing axes!
If forced to look at stereo images using diverging viewing axes for a longer time, pounding headaches are the inevitable result. Even if some people are capable of fairly amazing eye gymnastics, looking at stereo image should be enjoyable for every viewer. Therefore the projection rule has to be enforced under all circumstances in any stereo projection.
What limits and tolerances are allowed in stereo projection?
When distant objects are looked at, the viewing axes are parallel. If such distant objects are projected in stereo, a background screen separation of about 65-70mm equates to natural vision. Small increases are permitted. They should, however, not be more than 10mm for each 1m projection distance (resulting in an excess of 50mm over a 5m distance, 100mm over 10m distance and so on). The comfort of those in the audience seated closest to the screen must be considered. (For the experts: Technical literature indicates an angle of 1º is the maximum admissible divergence. However, it is the author's experience that many spectators suffer definite eye fatigue symptoms at this divergence!).
How do I adhere to the projection rule?
If the shooting and the mounting rule have been faithfully adhered to, not violating the projection rule is child's play. A stereo slide containing the infinity point is projected. The projectors are then moved sideways until the two infinity points on the screen (possibly plus additional tolerance value) are at a distance of 65mm. With this methodology the stereo window can quite easily appear to be located in front of the screen!
Only when projecting at home, with limited projection enlargement and for the sake of simplicity, may the stereo window be put exactly at the level of the projection screen. For projections in larger halls this procedure is not recommended, because 3-D viewing of nearby objects is much more enjoyable than of objects far away.
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