Journal of the Society of Motion Picture Engineers (1930-1949)

length and aperture setting, instead of a multitude of entries relating to all possible focus distances. But to gain these worth-while advantages, camera assistants would have to train their minds to udge distances nonlinearly, which would no doubt prove difficult !) In order to indicate how the general equation is derived, which connects the distance of objects in the scene with the distance of their corresponding images in the cinema from the spectator, it is necessary to return for a moment to the cinema and re-examine the parallaxes on the screen (zs). Referring again to Fig. 1, and adopting the sign convention that uncrossed parallaxes are positive and crossed parallaxes negative, we can see at once that when zs = t, P = F/0, i.e., image at NQ (infinity) zs = 0, P = V/\, i.e., image at N\ (plane of screen) zs = — t, P = F/2, i.e., image at N% (halfway out) zs = -2t, P = F/3, i.e., image at N3 (f of way out) Zg = — 3t,P = V/4, i.e., image at N^ (f of way out), and so on. Note: In our standard terminology, the letter t always represents the lateral distance between two optical axes, t itself denoting the separation of the human eyes (here assumed throughout as 2.5 in.), te the separation of the camera optical axes, tp that of the projector optical axes, etc. In other words, equal negative increments of parallax produce equal increases in N value. Moreover, these parallaxes are absolute; that is to say, they derive only from factors which are constant for any observer sitting in a given position in the cinema. They are irrespective of the size of the screen. But the corresponding parallaxes on the projected film, zp, are related to the screen parallaxes, zs, by an optical magnification, Af, which will be greater or smaller according as the screen is wider or less wide. Stated the other way round, a parallax of given magnitude on film will produce a greater or lesser stereoscopic depth according as the screen is larger or smaller. This important influence of screen size was first clearly stated, and its effects remarked on, by Professor Rule in the paper already cited. Depth Content in the Theater At this stage it will help to introduce another concept, that of the depth content of the film in the cinema ; in other words, the range of depth in space which the image occupies. Let us assume a difference in nearness factor of 2 between the front and rear planes of the image, which we shall express as *N%. Normally the range of N values would be from NQ (infinity) to Ar2, but from the point of view of the parallax analysis which follows, the position of the N range in space is immaterial. For example, A-/V2 might correspond to the range N\-N^ as in the recent McLaren cartoon film, Twirligig. Now it is apparent from what has been said that zs M (3) Since a change in N value of 1 results from a change of screen parallax of /, a depth content of ^N\ corresponds to a parallax on the projected film of 2.5/M in., a depth content of AjV2 to 5/M in., and so on. Magnitude of Film Parallaxes In order to give a more concrete idea of the magnitude of the actual film parallaxes when shooting for screens of normal commercial size, it may be helpful to deviate for a moment from the main course of the argument. Table II has been prepared to show the total span of film parallaxes available (in mils) for films Spottiswoode, Spottiswoode and Smith: 3-D Photography 255