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Chapter V
CINEMATOGRAPH LENSES
TECHNICAL terms used in photography are often puzzling to the amateur, particularly those terms which relate to the science of optics. The following glossary of optical terms has been prepared to give general information as to the descriptive words and phrases in ordinary use.
Equivalent focal length. Focal plane, is the plane in which a far distant object is imaged by the lens. The line drawn perpendicularly through the center of the lens is its Optical Axis; the point at which the Focal Plane intersects the Optical Axis, the Focal Point of the lens.
The Focal Length of a lens is the value upon which depends the size of the images produced by that lens. Its magnitude can be determined only by comparing the size of a given object with its image as formed by the lens. The distance of the object, unless very great, must also be considered.
For far distant objects the size of the image is in direct proportion to the focal length. A lens of 12-inch focal length will produce an image of a distant steeple twice as large as the image formed by a lens of 6-inch focal length.
Back Focus is the distance from the focal point to the rear surface of the lens. In very thin lenses, this back focus is equal to the focal length. In lenses of considerable thickness and in combinations of lenses, the back focus cannot be relied upon as any indication of the value of the focal length. The focal length of such a lens is equal to the focal length of a thin lens, which gives an image equivalent in size to the one formed by the combination lens, hence the term Equivalent Focal Length.
In using short focus cinematographic lenses it is important to know both the back and the equivalent foci, since the construction of some makes of motion picture cameras is such that the revolving shutter has not been placed close enough to the aperture to admit a lens of very short back focus without interfering with the shutter blades.
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