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Of in meters
70 in mefew
Fig. 4. Corresponding values of DI and Z)2 for determining stereo depth of field.
low, so that photographic possibilities will not be excessively restricted. As the maximum convergence differential, we therefore take 71 — 72 = 70'; this value, which seems reasonable for practical purposes, is in accordance with the stereoscopic projection German Standard DIN 4531 (July 1949, Beuth-Vertrieb GmbH, Berlin W15 and Koln).
In the case of cinematographic equipment, when the camera base is equal to the ocular base, the convergence differential is the same for the spectator viewing the image a' at a distance Dr = Xpfv/fp from the screen and for the photographer viewing a in nature while shooting.
We may therefore say that D\ and Z)2 are the distances from the camera to the boundaries A\ and At of the subject, within which the photographer can shoot without exceeding a visual convergence of 71 — 72 for the spectator at distance D ' from the screen.
We have
71 -72 -2b0(\/D, 1/JD,),
l/£i
(21)
If 7i 72 = 70' = 0.02 radians and 2b0 = 64mm,
we have l/Dj 1/Z>2 = 0.3125 (2~T)
when DI and Z)2 are expressed in meters (Fig. 4).
For example, if we are to photograph a subject whose most distant part is at
DZ = 5 m from the camera, the relation (21) shows us that no part of the scene photographed should then be less than D\ — 1.95 m from the camera.
This limit on convergence differential therefore determines a maximum picture depth as a function of the range — a depth of field, called the stereoscopic depth of field.
Stereoscopic Depth of Field and Position of Projector Windows
In a stereoscopic projection, the field of the image projected is bounded laterally and vertically by the projector windows. The two projector windows form a stereoscopic pair, and depending on the lateral position of the windows with respect to the centers of the images photographed, the resultant image will be more or less distant from the spectator. For example, if the windows were centered with respect to the images of the points at infinity^ along the axes of the camera lenses, the picture would seem to be located at infinity; with such a set-up we would always have D2 = oo, and no object could be photographed at less than DI = 3.2 m from the camera, otherwise the picture image would project beyond the stereoscopic depth of field.
Thus it turns out to be desirable to make the set-up such that the stereoscopic image of the projection windows seems to be an object located at 3.2 m from the camera. The frame then looks like
Eugene Millet: Depth Effect in Motion Pictures
521