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HANDBOOK OF PROJECTION FOR
is present all over the image. \t is not confined to the picture outline, but alters the relative width of things in different portions of the picture, as well as making the whole picture and everything in it abnormally tall. In fact this latter is the worst feature of the distortion. Outline distortion may be corrected by filing the aperture plate opening. Distortion of width of objects in the picture usually is not very noticeable, but added height is very readily discernible. Fig. 75 illustrates the effect of projection distance on projection angle, height of projection lens above screen remaining the same. If the lens be located 25 feet above screen center, and the projection distance 40 feet, the projection angle will be 32 degrees. With the projection distance 80 and 120 feet respectively, the projection angle is
HORIZONTAL LINE.
Figure 75
reduced to 17 degrees and 20 minutes, and 11 degrees and 46 minutes.
It is the opinion of the author that picture height offers the best and safest guide to permissible projection angle, and that any projection room location which increases the normal height of the picture by more than 5 per cent, is objectionable and should not be tolerated.
It will be observed that this permits an increase of the height of a sixteen-foot-wide picture by a trifle more than seven inches, so that at 80 feet projection distance a 15-degree angle would be within the limit by .2 of an inch. This would automatically add a maximum of 5 per cent, to the height of all objects on the screen, including the actors, so that a six-foot man who happened to appear just life size in