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 #:
000143
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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LENSES 119 The lens used should always have a focal length at least equal to the diagonal of the plate which it is intended the image shall cover. Thus for a half-plate (6J" by 4£") a lens of 8" focal length is required, while for a quarter-plate (4J" by 3 J"), one having focal length of 5 J" is suitable. This is an important point to remember if errors of perspective are to be avoided, for the longer the focus of the lens, the better and more truthful will be the picture produced. The focal length of a lens may be ascertained by focussing some very distant object on the screen, in which case the rays of light from the object to the lens may be considered parallel, and then the focal length may be taken as the distance of the screen from the diaphragm, if the lens is of symmetrical construction. The well-known relationship -+-=_ in which u = distance of object from lens, v= distance of image from lens, and /=focal length, may be made use of in the following simple way : — Let some object, the exact distance of which from the screen can be measured, be focussed ; then it can be shown that the focal length is equal to size of object size of image X distance of screen from object. /size of object V \size of image / for the ratio of the size of the object to the image will depend upon their relative distances from the lens, and will beU. V It So that above may be written -(u-\-v) M