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A PARALLEL OF 16 AND 35 MM. FILMS
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subject on the image plane than it will result from the use of a shorter focal length lens for the same image size.
A very interesting situation arises with regard to ways and means by which the exposure is most generally controlled. In professional cinematography for theatrical exhibition, the photographer is mostly striving for an exaltation of beauty. In other words, he disregards to a certain extent the truthful rendition of the object being photographed and strives to embellish it, also he strives to produce a picture the innumerable sequences of which can be viewed during over an hour's time with sustained attention without fatigue on the part of the spectator.
Excepting for scenes which have to carry an impression of strength of character, the professional cinematographer strives to avoid high "contrast" and to obtain a blending of highlights and shadows which eliminates harshness of contours without, however, disturbing good definition.
Fig. 2. Schematic drawing illustrating displacement of image for lenses of different focal length
a lens (L) of short focal length forms an image of an object (O) at (I). If the image plane is rotated for an ang'e (A), the center of object (O) is displaced from "d" to "d' " — A lens of double focal length forms an image at (I'). The object (O) will take position at (O" ) to form an image of same size as that formed bu the lens of shorter focal length. If the image plane (V ) is rotated for an angle (A') equal to (A), the center of the object (O') appears displaced from (D) to (D')
greater than "d d' ".
The effect is obtained mostly by photographing with a lens of large aperture at its maximum aperture, and controlling the exposure by means of a camera shutter of variable angular openings.
Since depth of focus of a lens is a physical factor inversely proportional to lens aperture and focal length of the lens, the professional cinematographer secures the desired effect by restricting the area of critical focus to the essential object planes. He operates with a lens of large aperture and reduces the shutter opening in order to control the admission of the light flux to the film.
Light filters are also used to a great extent for controlling exposures. Filters may be of neutral density; that is to say, filters that transmit all of the colored light radiations which compose either daylight or the artificial lights used in photographing, or selective filters, which absorb some of the radiations and permit a free passage of others.