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April, 1911.
MOTOGRAPHY
41
the motion picture in only one dimension. Both pictures are usually made the same height but vary in the width.
In the first place there is a constant ratio between the throw with its picture on the screen and the focal length with the film or slide. This may be expressed film or slide focal length
by the formula == . From
field throw
this we get the formula for the focal length of a lens.
largest dimension of film or slide X throw
Focal length = .
largest dimension of screen
To illustrate this formula : What size lens is required for a 64-foot throw picture 11x15 feet?
15 16'
-X64
60 = — = 4 inches focal length lens.
15 15
If we desire to have our stereopticon picture 11 feet high, what size lens must be used for it? 11
— X64 4 176
= =16 inches focal length lens.
11 11
The size of the screen equals
largest dimension of slide or film X throw
focal length
Example : What size screen is required in the following: throw 96 feet, 6 inches focal length lens?
15
-X96
16 90
= — =15 feet.
6 6
11/15 X 15 = 11 feet. Screen 11x15 feet.
It will be noticed that with the same throw, the size
picture increases as the focal length decreases, and
vice versa. Also, with the same focal length lens, the
given in the equivalent focal length (e. /.). This means the focal length of the combination of lenses. We sometimes see the phrase "%■ or K~size lens." These are photographic terms, and referred originally to the size of the plate covered by photographic lens. Their use in projection refers to the diameters of the lens mount; focal lengths up to ten inches being mounted in % plate, those over in Yz plate barrels. In stereopticon work, when using a lens over ten inches focal length, a much better picture can be had with a Yz size lens, although a J4 size one is generally furnished with the machine, unless otherwise specified. Condenser lenses are generall 4 or A1/?, inches in diameter and 6^2 or 7j4 inches focus. It does not make any difference which side of the lens faces the
light. The breakage of condenser lenses is due to unequal expansion or contraction caused by sudden extreme changes in temperature, and mounts which fit too tight. Square mounts are best as the lens may be quickly removed if broken, and they generally allow more room for expansion.
The outer surfaces of the condenser and projection lens should be cleaned daily. The projection lens need not be taken apart very often, as the inner surfaces of
Fig. 7.
picture increases in size as the throw increases. Do not select a lens of such short focus that the magnification will be so great that when the observer is near the screen the sharpness and perspective will be lost.
In all catalogues of projection machines tables can be found showing the size lens for various sizes of picture and throw. The size of the lens is generally
the glass do not get a chance to become dirty. Lenses should be cleaned with a soft cloth and not any rag that happens to be at hand. Never touch the surfaces of the lens with the fingers as you will leave a greasy impression that is difficult to remove. Lenses should not be exposed in a damp place, as moisture will collect on their surfaces.