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Kg. 7.
EH:
Schematic drawing of projection lens, showing f:1.6 lenses of Petzval type and polarizing filters. The optical axes are separated by 5.6 mm.
the two polarizers are oriented at right angles and at 45° to the horizontal so that the film can be viewed with polarizing spectacles available on the market, for example Polaroid 3-D Picture Viewer. The projection screen must preserve the polarization of light. A metallized screen coated with an aluminum-base varnish is satisfactory.
We saw, in our discussion of stereoscopic depth of field, that the projection window should be located in space near a plane 3.2 m from the camera. Suppose we are photographing a point A at 3 m from the camera with camera lenses 5.3 mm apart, focal length 12.5 mm, base 64 mm. The two homologous images A ' of A on the film will be
(64/3000)12.5 5.3 = 5.57
apart. The two projection windows should therefore be 5.57 mm or about 5.6 mm, between centers, in order for the plane of their stereoscopic image to appear to merge with the plane of A. The optical axes of the two projection lenses are likewise 5.6 mm apart. Under these circumstances, the planes at 3 m from the camera will coincide on the screen within 5.6 mm, regardless of the distance from the projector to the screen ; it would be possible to eliminate the projection windows and bound the picture with the edges of the screen itself.
The choice of focal lengths of the camera and projector lenses places the spectator at a distance of
D' = (1 2.5/20) ATP = 0.625*p
from the screen, or about two-thirds the screen-projector distance, for correct vision of the image.
Thus the Kern-Paillard instrument is a standard-base instrument in which judicious choice of focal lengths of camera and projector lenses affords vision approximating that of natural relief to a maximum number of spectators. The distances between the lenses are such that the image may be bounded either by means of the edges of the screen or by masking the projection lenses; in either case, the stereoscopic image appears to be bounded by a window about 3 m from the spectator. Anything photographed between 3 m and infinity appears behind this window; anything photographed between 1.5 m and 3 m appears between the window and the spectator. Photographing subjects closer than 1.5 m will result in emergence from the stereoscopic depth of field, and is inadvisable without the use of accessories which modify the convergence of axes and focusing of the Kern-Paillard system as they have been described in this article.
Eugene Millet: Depth Effect in Motion Pictures
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