Home Movies (1950)

3L .ABC* of Wlovu "THt CAMERA l£N$" by JASON WOODBINE ONCE YOU HAVE become reasonably familiar with your camera and its mechanical features, it is a good idea to turn your attention to what might be considered the very heart of the camera — the lens which forms the image. The camera contributes an important part, since it must keep the film flat in the gate, must move each frame accurately into position, and keep exposure constant from frame to frame, but all of these things are of little avail if the lens does not form a crisp, sharp image. Your first lens will undoubtedly be the one which comes as standard equipment on the camera. Knowledge of what it will and will not do will tell you what you can expect of that first lens. Later on, when you are more than likely to want to add other lenses to widen the range of things which you can do, a little understanding of the subject will help you to choose intelligently for suitability and worth. Actually, the term "lens" is a misnomer, and could almost be classed as slang, though it is widely used. The correct name for the image-forming element on your camera is an "objective." A lens is a single glass disc, which has one or both sides ground to a curve. A surface which curves outward is called convex, and one which is hollow is concave, while a flat surface is plane. A double convex lens, or one that bulges out on both sides, which was the earliest type to be made, resembles in shape the legume known as the lentil, and it is from that fact that the name "lens" springs. Consequently, it would only be correct to call the image forming unit on our camera a lens if it were composed of a single glass element. In the still camera field, a few very cheap outfits (and by "cheap" we mean costing a dollar or two) have had such single lenses. To the best of our knowledge, no movie camera, even the cheapest, has had such an imperfect optical portion. The reason, no doubt, is that the image on 8 or 16mm film is so tiny that it is of no possible use to anyone unless it is sharp, whereas a snapshot may be fairly satisfying even if somewhat fuzzy. If you want to see for yourself why we must have something more than a single "lens" if we want a sharp image, you can do some very instructive experimenting with a simply burning glass, such as woodsmen and Boy Scouts use to start a fire with the heat of the rays from the sun. In order to see the image which it forms, use a piece of ground glass if you have it, or if not, a thin sheet of paper. Standing inside a room, fairly well away from the light, face a window and hold the lens at eye level, with the ground glass or paper between the lens and your eye. Moving them back and forth, you will soon discover the position which gives you a fairly sharp The "lens" on objectives are the eyes of your film. — Protect them. image of the window and objects outside. At first glance (if you have never tried it) you will probably be surprised to sec how good the image is, but as you study it a little more you will begin to see serious flaws that make it quite unfit for anything as exacting as 8 and 16mm work. A single lens of this sort has seven prim try defects which mar the quality of th image which it forms, known as th. "aberrations" of a lens. While these are extremely interesting, it would carry us beyond the scope of a simple article to go into them in detail, but it is pertinent to mention the main effects or these aberrations. Briefly, an image formed by a simple lens has these major defects: a. Even in the center of the image, nothing is quite sharp, because the middle portion of the lens and the outer portion form their sharpest image at different distances from the ground glass (or film). At any distance we may set it, some of the rays will form a sharp image and other rays a blurred image of the same objects. b. Rays entering the lens at an angle will not form as sharp an image as those at right angles. c. Objects of different colors will not be sharp at the same time, and will not be of the same relative size. d. The sharpest image is formed not on a flat surface but on an imaginary curved surface to which the film cannot be bent. e. Straight lines in the subject will be curved in the image. All of these things add up to the fact that the image formed by a simple lens is hopelessly inadequate for the exacting requirements of home movie making. Hence, the objective on even the lowest priced cine camera consists of not one but several lenses. Three lenses is a minimum, and some of the more elaborate and expensive objectives have as many as eight or nine. Some of these are cemented together in groups, others are separated by an air space, but all must be very accurately centered in the same tube. This fact, basically, contains the answer to one of the amateur's most frequent questions: "Why are some objectives so expensive?" The answer is the complicated construction, the fantastic amount of mathematical work that goes into the design, and the practical difficulties of manufacture. The different lens elements are made of different types of optical glass, ground to different curvatures, spaced precise distances apart and all accurately mounted in the same tube. The design of a new lens is costly. Some have represented as much as 10-man-years of computing time. Once designed, tooling up is expensive, and manufacturing tolerances must be held to unbelieveably small limits. Once you have seen all of this at first hand, you will be surprised not at the high cost of good lenses but at the fact that they do not cost a great deal more. Some emphasis has been placed on this matter of cost, because it is important to realize that good lenses are worth what they cost. The optical business is highly competitive, and an objective priced out of proportion to its value does not survive long. There are objectives for 16mm cine work that cost two hundred dollars, and are • continued on Page 121 104