Richardson's handbook of projection (1930)

1230 HANDBOOK OF PROJECTION FOR LOCATION. — It is essential to good projection that the projection room location be such that there will be no objectionable distortion of the screen image due to the projection angle being excessive, and furthermore it is essential that the projection distance and picture size be so related as to require the use of a lens of such focal length as to combine critical definition over the entire field with reasonable optical efficiency. Your committer realizes that the great variation in theatre auditorium conditions demands considerable elasticity in the matter of projection room location, and therefore recommends : 1. That the relationship between picture size and projection distance be such as to require a projection lens of not less than four and one-half inches nor more than seven and one-half inches equivalent focus. 2. That within this range of lenses the projection angle be as nearly zero as is practicable, and in no case to exceed seventeen degrees. The recommendation as to the equivalent focus of the projection lens is based upon the observations of the members of your committee. The decision in the matter of the limiting angle was reached through a mathematical analysis of the situation. Let us refer to Fig. 442 in which O is the optical center of the projection lens, 6 is the projection angle, <x the vertical angle of divergency of the light beam, AC the height of the projected picture, and BD the height of an undistorted picture projected the same distance (OE) by the same lens. Realizing that for a given projection angle the distortion would be more serious with short focus lenses