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OFFSET PROJECTION
WILL G. SMITH
When a projecting machine is set in any other position than directly opposite the center of the picture screen, in other words, when the projecting machine is "offset" from the center of the screen, certain highly undesirable effects are produced in the picture. These undesirable effects are of two kinds, first, blurring of certain edges of the picture and, second, distortion of the objects appearing on the screen.
Taking up the first of these effects, blurring of the edges, this is seen to be due to the fact that one edge of the screen is further from the lens than the other edge, and, consequently, if any chosen part of the picture is "focused" clearly on the screen other parts of the picture must, of necessity, be "out of focus." When the "offset" of the projecting machine from the screen center is not greater than, say, 5° to 10° then the blurring effect is not particularly noticeable, but when, as in some theatres in present use, the angle is as great as 35° it is utterly impossible to produce a clear picture of any attractive size.
The second of the ill effects of "offset" projection, the distortion of the objects shown, is due to the fact that the light beam coming from the projection machine to the screen is cone shaped and grows rapidly larger in area as the screen is approached. Consequently, if a section is taken through such a light cone at an angle, then, since one edge of the section plane cuts a narrower part of the beam than the other edge, it follows that one edge of the picture will be larger than the other; and further, since the further the cutting section is taken from the lens the larger the projection is, it follows that the same object moving to different positions in the picture will undergo a change in size.
Considering now a special case, which is perhaps the most common case of "offset projection," let us consider that the projecting machine is placed in the balcony of a theatre directly in line with the center of the screen on the stage but considerably above it. Then the light cone will come down to the screen and strike at an angle and the following undesirable effects will be apparent:
1. The top and bottom of the picture will be blurred while the middle will be clear.
2. A body moving from the top to the bottom of the picture will become considerably larger.
3. All vertical objects will be large at the bottom and small at the topi — for instance, a man will have big feet and a small head.
4. All objects will be abnormally tall.