Handbook for motion picture and stereopticon operators . he box it transmits momentary images tothe moving sensitive film, and as the film is con-stantly wound upon the drum each picture is takenin a new place upon the film. The lenses travelin an arc of a circle, to be sure, but the opening isso small, as compared with the lens circle, and thearc so nearly coincides with a straight line thatthis feature is not detrimental, neither is the fact 24 Motion Picture that the negative and the lenses are at slightlydifferent distances from the subject. The resultof continued operation is a length of
Handbook for motion picture and stereopticon operators . he box it transmits momentary images tothe moving sensitive film, and as the film is con-stantly wound upon the drum each picture is takenin a new place upon the film. The lenses travelin an arc of a circle, to be sure, but the opening isso small, as compared with the lens circle, and thearc so nearly coincides with a straight line thatthis feature is not detrimental, neither is the fact 24 Motion Picture that the negative and the lenses are at slightlydifferent distances from the subject. The resultof continued operation is a length of negative filmwith similar pictures thereon, each picture differ-ing slightly from the preceding one owing to theconstant movement of the object hundred and fifty pictures per second havebeen made in this camera and there is no limit tothe speed, except illumination and sensitiveness offilm. Lenses. The choice of a lens for a camera should begoverned by the focal length, together with therapidity and depth of focus, it being understood,. of course, that the lens is free from astigmatismand from chromatic and spherical aberration. Theshorter the back focus the greater the depth offocus; and the larger the proportion of the aper-ture to the equivalent focus the greater therapidity. Operators Handbook 25 Stops. Most photographic lenses are marked in fnumbers, on what is known as the U. S. sys-tem, that is, the aperture is expressed as a fractionof the equivalent focus. Now with a given lens astop with an aperture of twice another has an open-ing four times the area, admitting four times thelight, so that the exposure for the latter would befour times that through the former to get the sameexposure. The converse is also true, that a nega-tive twice as far from a given aperture requiresfour times the exposure, because the light on theplate or film in the second position is spread overfour times the surface in the first position. Alens of 4-inch equivalent focus and w
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