The advance of photography : its history and modern applications (1911)

Record Details:

Title:
The advance of photography : its history and modern applications
Date:
1911
Publisher:
London : K. Paul, Trench, Trübner & Co., Ltd.
Volume:
The advance of photography : its history and modern applications
Leaf #:
000142
Language:
English
Subjects:
Motion Pictures, Motion pictures--Technology
Collection:
Books
Format:
Books
Contributor:
Media History Digital Library
Sponsor:
Media History Digital Library
Coordinator:
Media History Digital Library
Description:
Date Added:
November 02, 2023
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118 THE ADVANCE OF PHOTOGRAPHY and are slow in action. It must be remembered that all three classes of lenses are rectilinear. There is still another system of lenses which is of great use in obtaining views of inaccessible objects, I refer now to the Telephoto-lenses. In all lenses of this class there are two systems of lenses, the front one being positive, having a large aperture and short focus, the hinder one negative and of less focal length than the front. The general idea of the principle of this system of lenses can be obtained from the following diagram. The positive lens A B would bring the light to a focus at C, but the negative lens E F causes these rays to diverge slightly, and they so come to a focus at D. Hence they appear to behave as if they proceed, not from A B but from a new position, G H, i.e. the apparent focal length of A B is increased, and the image obtained magnified. The distance G to D, or more exactly the perpendicular distance of D from the straight line joining G to H, is known as the equivalent focal length of the lens system. This diagram does not show the actual lenses used ; for this see p. 129. Terms in General Use. — After briefly considering one or two terms which we have constantly to use with respect to photographic lenses, these various systems of lenses will be considered, for that purpose reference will be made to some of the best-known lenses.