International projectionist (Jan-Dec 1945)

FIGURE 2 such as the crater of a carbon arc — the rays of light will naturally spread out in every direction and, therefore, will be inherently, and without action of any lens, diverging rays of light. If the light comes from a large and remote source, such as the sun, those rays of light that reach the earth are effectively parallel to each other. No lens or mirror is needed to make them parallel. Now, if parallel rays of light pass through a converging lens, the action of the lens will be to converge them to a point, or focus. The rays will come together at the focal distance of the particular lens in question. This action is diagrammed in Figure 2. The action is reversible. If a point source of light is placed at exactly the focal distance from a converging lens, the action of the lens will be to make the rays parallel rays after they leave the lens. Each lens has its own focal distance, according to its curvature. Conjugate Foci Assume a point source of light is located, in Figure 2, farther away from the lens than point F of that figure. Then the rays of light after leaving the lens will not be parallel to each other, but will converge into a point, or image of the original. The place where the point source is located, and the place where the image point is located, are called the conjugate foci of the lens. To the contrary, if the point source is moved nearer to the lens than F, the rays of light will not converge on the other side of the lens, even though it is a converging lens: they will diverge instead. All this is made plain by Figure 3. The heavy horizontal line in A of Figure 3 represents the optical axis of the lens, and the axis of the optical system involving that lens. A vertical line is drawn through the lens, crossing the optical axis at the lens optical center. Now along the optical axis four points are marked, 2F and F at the left of the lens, and again F and 2F at the right of the lens. F is the focal distance, which is the same at either side of the lens, the action being reversible. 2F is exactly double the focal distance. The arrow drawn at the extreme left of Figure 3A may be taken as an image to be enlarged, or the tip of the arrow may be considered a point source of light. Referring to Figure 3A, if the arrow is further from the lens that 2F, the rays of light will be brought to a point, an image, on the right side of lens somewhere between F and 2F. The location of the arrow and the location of the image formed on the right side of the lens are conjugate foci. If the arrow is placed exactly at 2F (Figure 3B) the image on the right side of the lens will be formed exactly at 2F. If the arrow is placed between 2F and F (Figure 3C) the image will be formed beyond 2F. Also, if the arrow were placed at F. no image would be formed except at infinite distance — that is, the rays of light would emerge from the lens parallel to each other, as in Figure 2. If the arrow is placed closer to the lens than F (Figure 3D) the rays of light will diverge as they leave the lens. No image will be formed at all; but there is a "virtual" image formed at V. A simple and useful formula will show where the image (not "virtual image") will be formed in each case: 1/od + 1/id = 1/fl where od is the object distance, id the image distance and fl the focal length of the particular lens used. Projection Lenses The simple lens action diagrammed in Figures 2 and 3 is based upon bi-convex lenses, as shown in those diagrams. The operation of a plano-convex lens would, however, be exactly the same, and just as much reversible. Bi-convex and planoconvex lenses are widely used as condenser lenses in projection, when they are so located with respect to the point source of light provided by the crater of a carbon arc as to converge the light rays toward the projector aperture. Concave and meniscus lenses, as shown in Figure 1, are seldom used singly in projection work. They are, however, found in every projection room as "elements" in a compound lens; for the projection lens invariably is compounded out of convex and other simple lenses. The function of the projection lens is to treat a frame of film as if it were the arrow of Figure 3C. forming an enlarged image thereof on the projection screen. Figure 4 shows the general construction of a compound lens, as used in mo tion picture projectors. The object of compounding lenses is to reduce optical distortion, or aberration, as it is correctly called. If a simple lens, as shown in Figures 2 and 3. were used for projection (it could easily be done) two types of aberration in particular would result. One is called chromatic aberration, the other spherical aberration. A simple lens tends to break up white light into its component colors — not as thoroughly as a glass prism would do it, FIGURE FIGURE 4 but thoroughly enough to create rainbow effects or colored halos, particularly near the edges of the projection screen. A simple lens also introduces distortion of the image, also near the edges of the screen to a large extent. Lenses of different shapes, and of different kinds of optical glass, produce different distortions or aberrations, and it has long been known that the aberration created by one lens can be effectively cancelled out by an equal and opposite aberration produced by a lens of different shape or different material. The compound lens does not have a "focal length" as such; each of its components has its own focal length; the compound lens has an "equivalent focus" or E.F. which in practical work so far as the projectionist is concerned amounts to the same thing. Thus a compound lens with an E.F. of 4" behaves exactly as if it were a simple lens of 4 inches focal length — but with far less aberration. While the designing of complex lenses is no business of the projectionist, but an extremely skilled and complex specialty in the field of optics, it is important for ihe projectionist to know that the spacing of the elements in a complex lens is critical. Some of the elements are not spaced, but cemented together with Canada balsam. For these reasons a compound lens should not be opened except in extreme necessity, preferably not at all; and great care should be used in applying any cleaning fluid to the outer surfaces, since if such fluid leaks into the assembly it may dissolve the Canada balsam. Then the lens would have to go back to the manufacturer to be re-built. Projection lenses are commonly rated in terms of their E.F., and an order is placed for a lens of so many inches E.F. In other fields, however, lenses are rated 8 INTERNATIONAL PROJECTIONIST