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Data: M = 218, /. W = 15 ft, and
2.5 W = 450 in., .-. A = 1,125 sq in. Z>! = 44p, /. 48P = AW jc = 50 mm = 1.97 in., te =
1.25 in., /. C = 536.9 sq in.
From Eq. (14),
536.9 X 44
B
6,000 = 1.44 in.
-2.5
Hence, from Eqs. (11), (12) and (13),
2.10 X 2.34 = 2.00 X 1.53 = 4.9/3.1 = 1.6
md
4.9 3.1
From this it is safe to conclude, and can indeed be observed in the film, that some elongation of the side of the face and the shoulders will be noticeable, especially if the dancer turns so that the same features undergo differing magnification in quick succession. The more interesting case in which the dancer pirouettes with outstretched arms would call for integration, since the depth occupied by the arm stretching toward the camera would be nonlinearly magnified in a B-{ system. This difficulty can be overcome by employing an extension of the technique next to be described.
Interpreting the Stereotest
A graphical presentation, based on the principle of Fig. 3, greatly simplifies the study of scenes in space. So long as the distance of a plane in the scene from the camera is known, its position in space in the theater can be read off in an instant, together with the parallax which a point in this plane has produced on the camera negative, and finally the convergence and interaxial separation which obtained when the scene was shot. Figures 8 and 9 are the representation of Slates 35 and 1 5 respectively in The Black Swan. After development, synchronized left and right-eye frames of the two Stereotests are cut from the negative tracks and placed
under a special Swift traveling microscope with pilot-pin registration. By placing the two frames successively on the same pins, the parallax between corresponding points on the tests, at each distance, can be read off with great accuracy. These results from the actual film may be tabulated as in Table IV.
Table IV
zc (mils) 40p 22p
Slate 35 Slate 15
-4.3
-2.2
+7.2 +6.4
It should be emphasized that, because of the extremely small parallaxes needed to produce a 5 N of only 0.1 (see Table III), the most painstaking efforts must be made to keep microscope reading errors down to the lowest possible limits. Figures 8 and 9 show how the 40 and 22 p points are plotted on a graph which represents zc on the *-axis and distances in p on the jv-axis. Since, on such a graph, B — 0, B+ and B— systems are equally represented by straight lines, it is only necessary to lay a ruler between the 40 p and the 22 p points. The resulting line represents all the stereoscopic relationships in the negative, and the *-axis can of course be additionally graduated in N values for any assumed value of M by using the relationship,
N=\-^ (-11)
In Figs. 8 and 9 this has been done for M = 218, the magnification for which the film was shot.
With the aid of a special protractor (not shown in the diagrams), it is possible to read off with great accuracy the actual values of tc and <p (or h) which obtained when shooting. These results may be tabulated as in Table V.
No practical technique is imaginable for changing the magnitude of the C factor after shooting (assuming that M re
Spottiswoode, Spottiswoode and Smith: 3-D Photography
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