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Fourteen
The INTERNATIONAL PHOTOGRAPHER
September, 1929
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-BY
JOSEPH DUBRAY, Manager of Special Service Department, Bell & Howell Camera Co.
Sound pictures have brought about the necessity of making a simultaneous use of lenses of varied focal length, and it is, therefore, quite essential for the cinematographer to be able to rapidly determine with a fair degree of accuracy the angle of view covered by any lens that he may be called to use.
A very simple formula will permit to rapidly determine the space-width covered by lenses of any focal length and at anv distance from the camera.
The lens used is of a focal length of 50 millimeters. The equation becomes
24 X
W =
50
-2L = ii feet
In the case in which the reduced sound aperture is to be considered, the equation becomes
w=
where again:
20 D
affords a much easier mental calculation and the results obtained are sufficiently accurate in actual studio practice, especially when rapidity is more essential than absolute precision.*
It is quite evident that whenever two factors of the equation are known, besides the constant width of the camera aperture, the unknown factor can easily be determined.
If the width of the image space and the focal length of the lens are known,
L£NS
NODAL PLANES /,'
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Two main cases present themselves for our consideration. First, the case in which the standard full size camera aperture is used, and, second, the case in which the smaller sound aperture is required.
In the first case the formula may be expressed as follows:
24 D
W = —
w F
in which:
W represents the width of the object space.
24 is the width of the photographic image in millimeters.
D represents the distance of the object plane from the lens.
W represents the width of the object space.
20 is the width of the photographic image in millimeters.
D represents the distance of the object plane from the lens.
F represents the focal length of the lens in millimeters.
For example: Suppose that the camera is set, as above, facing a wall and 23 feet away from it and that the lens used has a focal length of 50 millimeters, we will have
20 X 23 W = tn = 9.2 feet
the distance at which the camera is to be set will be found by:
W F D=-^4~ for the full standard aperture
WF
50
D=
20
for the sound aperture.
For example: Suppose that the width of the object space to be covered is 63 feet, a lens of 40 m.m. focal length will demand that the camera be set at:
D =
63
X 40 24
105 feet
It may be noted here that the true width for the full standard aperture
F represents the focal length of the lens in millimeters.
For example: Suppose the camera is set facing a wall 23 feet away from it.
of the sound image is, as per the recently proposed Standards, of 20.955 m.m. or very nearly 21 m.m. We suggest, however, the 20 m.m. constant because it
D =
63 X 40
20
for the sound aperture.
126 feet