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290 PRINCIPLES OF CINEMATOGRAPHY
produced on Ike screen in the 35-mm theatre, as shown in diagram *A\ In terms of angular rotation as experienced by the average viewer, the unsteadiness experienced in case 'A' will tend to make the eye move through 1 • 2 minutes (just over 1 /60th of one degree) when 'resolving' this vibration. In case 'B' screen movement of equal magnitude will tend to make the eye move through 2-6 minutes, or more than twice that encountered in case 'A'. A comparison between the two important angles is shown in diagram 'C, Figure 127, and it should be obvious that the average 16-mm viewer will experience greater picture unsteadiness than the average 35-mm viewer.
The arrangement of the seating is, of course, dependent upon the picture size, regardless of the film size or projection apparatus, and is so arranged to ensure that the picture will subtend the same angle to the average viewer in all cases. If, therefore, picture unsteadiness is expressed as a percentage of the total picture height, it is seen that, in the case of the 35-mm film, this amounts to 0 • 1 62 % whereas, for the 16-mm films, this figure increases to as much as 0-353%.
If, therefore, it were possible to produce 16-mm reduction prints with a final accuracy equal to that recommended for 35-mm release prints, and also to obtain a projector as accurate as a professional machine, the resultant screen picture, when viewed from the correct distances, would show greater unsteadiness than the 35-mm picture viewed under similar circumstances and produced to equal accuracy. The unsteadiness of the 16-mm picture would exceed that of the 35-mm picture as 353 exceeds 162, that is, it would be 21 times greater. This amount is, of course, the ratio between the height of a 35-mm projector aperture and a 16-mm projector aperture. On first consideration it may be thought possible to overcome this difference by placing the audience at a greater distance from the screen. Quite apart from reducing the apparent size of the picture, such a move would require placing the audience 2-1 times further from the screen, that is, in the example given, extending the distance between the screen and the closest observer to 20-feet.
Clearly the foregoing suggestion is quite impracticable and, as a basis of consideration, a screen movement of 0-350-inch at a magnification of 350 times must be accepted, assuming no losses of steadiness in the projection mechanism. In other words, we must aim to print successive pictures onto the 16-mm film within a total tolerance of 0-001 -inch, or plus and minus 0-0005-inch, when measured at the centre of the picture area.