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104 Transactions of S.M.P.E., July 1927
small area even when the aperture of the lens is nearly half its focal length.
In lantern slide projection on the other hand the magnification required is usually not more than a third of that required for motion picture projection and therefore the matter of high aperture is of relatively less importance. The field of view to be covered, however, is often larger. These requirements are better met by a lens of smaller relative aperture than the Petzval and one with considerably greater covering power, i.e., ability to form an image of satisfactory quality of a fairly extended object. Such a lens is the triple anastigmat. It performs excellently and is relatively inexpensive. It maj^ be of interest, however, to point out that lantern slide projection from a motion picture projection booth involves no greater angle of field of view than is required of the motion picture projection lens and such projection is often satisfactorily accomplished with a single achromatic lens or a telescope objective.
Up to this point no reference has been made to the influence of the type of illumination employed upon the demands made upon the projection lens but the implication was made a few minutes ago that it might not always follow that increased illumination would be gained by increasing the relative aperture of the projection lens. In fact, in order to insure that an increase in the relative aperture of the projection lens shall be effective, it is necessar>^ to see that the aperture of the illuminating system be sufficiently large.
In order to pursue this matter a little further it will be convenient to introduce the expression "angular aperture." If an eye be placed at the center of the film gate and turned towards the objective, the largest circle of light visible subtends at the eye an angle which we call the angular aperture of the objective. Similar 1}% if the eye turns towards the illuminating system the largest circle of light seen subtends at the eye an angle which is the angular aperture of the illuminating system. Now, in so far as the central point of the film gate is concerned, there is no gain in illumination if the angular aperture of the projection lens is increased to a value greater than the angular aperture of the illuminating system. Now, the angular aperture of the illuminating system is a complex thing. In addition to the physical dimensions of the optical units of which it is composed it depends upon the size of the light source and upon the location of the image of the light source with respect to the film gate. The illustration in Fig. 3 will help to illustrate this point. Here a motion picture pro