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332 CINEMATOGRAPHIC ANNUAL
The lens mostly used in 35 millimeter motion picture photography is one having a focal length of 50 millimeters and the angle of view covered by it is very nearly equal to that covered by a 25 millimeter lens on 16 millimeter film.
The perspective effect of the projected picture, that is to say the apparent distance between points in the several depth planes of the object space is dependent upon three factors: 1 — the focal length of the photographic lens; 2 — the focal length of the projection lens; 3 — the distance between viewer and screen.
A picture taken on 1 6 millimeter film projected with a 2" projection lens and viewed at the relatively short distance at which such pictures are generally viewed gives the same average effect of perspective as a picture taken with a 2" photo lens, projected with a 4" projection lens and viewed from the average distance between spectator and screen in a theatre.
It is true however, that in professional cinematography destined for exhibition in theatres, the use of a much greater range of focal length lenses has become essential in recent times.
This is due to the fact that "impressionistic shots" are being more and more used and very short focal length lenses adapt themselves admirably to these "effects" and that (this has been recognized for several years past) "close-ups" bordering portraiture work, are more pleasing when taken with a lens of a longer focal length than the 2", usually a 3". Two main reasons may be ascribed to this increase in pleasing effect, first a 3" lens gives a better balanced sense of perspective for the face of the subject, and second, that a 3" lens worked at the same aperture as a 2" has less depth of field than the latter. The backgrounds are therefore more diffused which enhances by contrast the sharpness of the modeling of the face of the subject and focused the attention of the viewer to it.
Users of 16 millimeter film can and should follow the same trend of thought that regulates the choice of a lens for the professional. Very short focal length lenses will be found very useful for obtaining a picture record wherever space is lacking for setting the camera to include an angle of view especially desired.
For close up motion picture portraiture the amateur cinematographer will find a 2" lens ideal, and longer focal lengths can be used with equal efficiency by both professionals and amateurs wherever they wish to reach inaccessible subjects, be they distant views or subjects of dangerous approach
The user of 16 millimeter cameras must however hold in mind that whenever he dispenses with the use of a tripod and takes his pictures holding the camera in his hands (which is seldom if ever the case for the user of 35 millimeter film) , he will find that the use of very long focal length lenses is quite inadvisable because of the practical impossibility of holding the camera with sufficient firmness to insure a steady screen projection.
A long focal length lens is in practically all cases used for photographing objects at a great distance, and the slightest motion of the camera results in a much greater displacement of any point of the