American cinematographer (Jan-Dec 1946)

A Positive VariFocal View Finder For Motion Picture Cameras The author, Frank G. Back, M.E., Sc.D., is associated with the Research and Development Laboratory, New York. Presented at recent Technical Conference of Society of Motion Picture Engineers, and published in Decem¬ ber, 1945 issue of SMPE Journal. Reprinted by special permission. By FRANK 6. BACK A new positive vari-focal view-finder for motion picture cameras eliminates the disadvantages of the two classes of finders now in use. The new view-finder uses only positive elements, thereby pro¬ ducing an upright real image. And it is possible to secure this real image in a frame of constant size. The image be¬ comes variable over a wide range along a smooth and continuous curve instead of by intermittent steps as in the case in some other types of finders. The two classes of finders presenfly used secure images by two different methods. One limits the fields of dif¬ ferent camera lenses by mechanically changing the size of the frame which surrounds the image. The second class changes the field by optical means. In the first class, we see that the image frame gets smaller as the focal length of the camera objective increases. The disadvantages of this are apparent. Those finders of the second class which change the field optically do one of two things: they change the frame size, as in the first class, or they produce only virtual images which do not frame dis¬ tinctly. Negative-type finders, as de¬ scribed, also have large parallactic error. This is particularly true when the eye is displaced. One other type of view-finder having a turrethead might be mentioned here. Its evident disadvantage lies in the limitations placed upon it by the number of optical elements in its turrethead. The vari-focal system of the new posi¬ tive view-finder is based, of course, on the principle of the astronomical tele¬ scope. The finder adds to this principle certain other features which give it de¬ cided advantages. In the astronomical telescope, magnification is determined by the ratio of the focal length of the front lens combination to the focal length of the rear-lens combination. Thus, a long focus front lens combined with a short focus rear lens gives us magnification. Fig. 1 (a) illustrates this clearly. Conversely, a short focus front lens coupled with a long focus rear lens brings about reduction, as seen in Fig. 1 (b). The new positive view-finder uses a front lens and a rear lens of approxi¬ mately equal focal length. The front and rear lenses do not move and are known as “stationary lenses.” The variation is obtained by two lenses of shorter focal length mounted in a barrel with the view-finder housing between the two stationary lenses. These two shorter focus or “variator lenses” can be moved forward and backward from the front stationary lens to the rear lens. The movable barrel in which they are mounted is called the “variator.” This is how the variator operates: The stationary, and variator lenses are com¬ puted in such a way that when the vari¬ ator is in the extreme front position, the combined power of the stationary front lens and the first variator lens is such that the inverted real image produced by them falls on the second variator lens. Fig. 2 (a) shows this. Thus the second variator lens acts as a field lens in this position and does not participate in the forming of the image. So far, the vari¬ focal system acts as a telescope that has a short focus objective and a longer focus rear lens, producing a reduced image covering a wide-field angle, as with a wide-angle lens. (Continued on Page 101) IMACf £»ECT0» *SS£M«iv E*ONT LENS / Vif'l'flNOC* SETTING *SSEUSlr Fig. 3. Fig. 4. 84 March, 1946 • American Cinematographer