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Distance from camera, (rhos)
60
Plane of dancer's fingertips
Curve (A)
Curve (B)
55
50
Plane of dancer's shoulder
40
N! 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 N2
Fig. 13. Enlarged section of Fig. 12, with "human vision" rendering of Slate 15 (Curve B) compared with shot in film (Curve A). N values may be converted to image distances from spectator (P) by Eq. (1) (P = V/N}. Assume spectator seated at 2.5W, i.e., 450 in. from 15 ft screen. Let P = image distance to dancer's shoulder, P' to her fingertips, when her arm is outstretched to the spectator. Hence, stereoscopic length of dancer's arm = P — P'.
From Curve B, "human vision," P = 450/1, P' = 450/1.86, .-. P P' = 208 in. From Curve A, shot in film, P = 450/1.13, P' = 450/1.52, .-. P P' = 102 in.
The general applicability of a "human vision'.' technique can perhaps be most quickly judged by a statistical summary of the te values employed in some recent pictures. The settings indicated by calculators such as the Stereomeasure and the Polaroid Interocular Calculator naturally give no preference to the value of 2.5 in., which is merely one setting in a wide and infinitely divisible range; but if, to give "human vision" all permissible latitude, we assign it the whole span of tc values between 2.3 in. and 2.7 in., we can analyze its limitations in the light of actual tc figures from productions. Picture A is a studio film shot in sets of normal dimensions ; Picture B is a documentary film shot principally out of doors.
Table VII. Number of Shots Lying Inside and Outside the "Human Vision" Range for Two Typical Productions
Less More
than 2.3 than 2. 3 in. 2.7 in. 2.7 in.
Total
Picture A Picture B
16 6
20 10
1 29
37 46
Thus 46% of the shots in Picture A, and 76% of those in Picture B, fall outside the "human vision" range when calculated according to the general theory, and without any preconception that t must always equal tc. It must be remembered that the scene we have been discussing in such detail is not in any way abnormal, but is representative of in
Spottiswoode, Spottiswoode and Smith: 3-D Photography
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