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108 RICHARDSON'S BLUEBOOK OF PROJECTION
Simple Lens
(36) A single, uncorrected lens.
Spherical Aberration
(37) That quality of an uncorrected lens that focuses light rays passing through its various zones (at different distances from its axis) at different distances from the optical center of the lens. The further from the axis a ray passes through, the closer to the lens it will be focused.
Working Distance
(38) As applies to a projecton lens, the distance from surface of photographic emulsion on film, when it is in position over the projector aperture, to rear or first surface of the lens.
(39) There is a difference between "working distance" and "back focus, " with which it is often confused. "Back focus" is a technical term indicating the distance of film emulsion to first surface of the lens when light rays illuminating the aperture are parallel — a condition never found in projection. Back focus therefore is a term that cannot be used correctly in the lexicon of projection optics. •
Lens Action
(40) The action of lenses upon light is based upon the principle that if a ray of light, on passing through a transparent medium of one density encounters the surface of another transparent medium of heavier or lighter density, at an angle, and enters it, the course of the ray will be altered. How much the ray will be "refracted" in passing at an angle into the medium of different density depends upon (a) the difference in density of the two mediums and (b) the angle at which the surface of the second medium is met.
(41) From the optical engineer's viewpoint each tiny pinpoint of a curved lens surface presents an entirely separate proposition from every other pinpoint of the lens surface, even though the two points join each other. Each point presents a different angle to light rays than