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64
BETTER THEATRES
August 2 1, I 943
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Exhibitors of America have many duties to perform these war days. You build unity and morale through motion picture presentations— and you promote and support the various government drives that are initiated to spur war production and civilian defense.
RCA Service, like exhibitors, is carrying on important war duties: RCA engineers are rendering scheduled service to projection room equipment in thousands of theatres to "Keep 'em Running"and other RCA Service groups are installing military equipment and instructing personnel, in this country and at the battlefronts.
The RCA Service organization is today more than nation-wide ... it is world-wide . . . serving the home front and battlefronts too!
RCA SERVICE CO., INC.
RADIO CORPORATION OF AMERICA
Subsidiary
Camden, N. J.
The
LIGHT on your
om SCREEN
By CHARLES E. SHULTZ
Member SMPE . . . Honorary Life Member LOCAL 365, IATSE & MPMO
The Practical Meaning Of Relative Aperture
Projectionist Roy Brierly of I A Local 306, New York City, asks :
"I have often heard the relative aperture of a lens referred to as the f :speed of that lens. I know that the relative aperture means the relationship between the focus of the lens and its smallest opening, but I would like to know how the word 'speed' is associated with the relative aperture,
15 foot-candles without film, when this lens had a relative aperture of f :2.2, what percentage of the light would pass through this same lens at a relative aperture of f:2.5, or f:3, and what would be the size of the smallest opening in the lens in each case?"
This is a very good question on an important phase of lens operation which often proves puzzling and I am sure it will interest many other projectionists.
In answer to your first question regarding "f:speed": You are correct in your belief that this is a term for one of the fac
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WATER TANK
which is a factor designating how much light is transmitted by a lens and not how fast it passes through the lens.
"I wonder if you would also tell me the percentage of difference in transmission for the same lens at several different values of relative aperture. In other words, if I were using a 4I/2-inch E.F. lens and I had an average light all over my screen of
tors which determines "how mueh" light will pass through a lens.
The use of the word "speed" undoubtedly originated from the use of photographic lenses in which high relative aperture permitted a greater shutter speed for the same exposure value because as much light was transmitted in a short interval of time as that transmitted by a lens of