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54
W. B. RAYTON
[J. S. M. P. K.
maker. There is one possibility, however, which leaves the film manufacturer free from embarrassment in so far as his emulsions are concerned but which still depends for its success on the lens designer. The method referred to consists in enlarging the picture area without changing the focal length of the lenses. This, translated in optical language, calls for both photographic and projection lenses of larger field of view. For the sake of any who may be unfamiliar with the meaning of the term field of view it may be well to explain that the quotient of half the diagonal of the picture area divided by the focal length of the lens is the tangent of half the angular field of view.
AO *w
Referring to Fig. 2, — = tan — where / is the focal length of the / 2
lens and w is the angular field of view.
The commonly used focal lengths in motion picture practice run from 40 mm. to 150 mm. Lenses both shorter and longer are used on occasion but not frequently. The following table presents the values of the angular fields of view demanded by three different picture areas for lenses within these limits.
Table of Angular Field of View
Focal Length of I/ens
19 X 25 Mm.
Picture Area 18 X 36 Mm.
23 X 46 Mm.
40 mm.
42° 52'
53° 24'
65° 28'
50
34 52
43 50
54 26
75
23 38
30 02
37 50
100
17 50
22 46
28 50
Beyond doubt the most popular lens in motion picture photography is the 50 mm. lens. On standard film the field of view covered is slightly less than 35°. For the 23 X 46 mm. picture area the field covered is practically 54V2°. This is not by any means an unheardof angle in lenses of relative aperture of //4.5 or even//3.5 but no photographic lens appeared to be available with sufficient speed and satisfactorily sharp definition to cover a picture size 23 X 46 mm. at the time this size was first attempted. I am happy, however, to announce that I have been able to design a 50 mm. lens of a relative aperture of //2.3 which covers this area very satisfactorily. An attempt has been made in Fig. 3 to indicate the quality of its performance. To cover the field with lenses of longer focal length is a task of less difficulty, but here one must guard against a deteriora