International photographer (Jan-Dec 1941)

l6lVIM. dEpARTMENT While Thinking about Mr. Unseld's article on Lens Perspective in this department last month, we began wondering whether we had not jumped a little ahead of ourselves. Frequently, in this and other technical fields ,we are prone to take many basic established facts for granted without understanding why they are basic facts, and to work on from there, often achieving a high degree of proficiency with a medium about which we are none too familiar from the "why" standpoint. Generally speaking, the amateur who is seriously interested in cinematography thinks of his lens as an integral and important part of his camera and that is either good, bad, or indifferent, according to the price that he was able to pay for it and the job that it has to do. He devotes most of his time to doing those things. However, there is much to be said in favor of knowing why those things that take place do, aside from the feeling of personal satisfaction that comes from knowing the subject thoroughly. This can all be summed up in the statement that this knowledge removes the mystery of why any particular shot was a success or a failure due to optical reasons. This article does not pretend to remove all that mystery for the obvious reason that a subject of such scope could not be treated completely within these limits. It will, however, give a basic understanding of the principles involved that will form a foundation for further reading. Lens action is based on the principle in physics known as refraction. Reduced to everyday English, it is the bending of light rays. We are all familiar with the sight of a spoon in a glass of water or tea, where the spoon appears to be bent at the surface of the liquid. We have watched an object lying at the bottom of a shallow body of water and seen it change its shape — become elongated, shortened, or otherwise distorted — as the ripples of water flowed over it. And we have watched the "heat-waves" rising from the surface of the highway while driving along and seen the "ripples" make the distant objects appear to be fluttering in the "waves." These are all everyday instances of refraction. Refraction takes place when a ray of light passes through one or more mediums of varying density, or when a medium of a single density varies in thickness and its surface beemnes curved. In the case of the water and I he spoon, the water is one medium and air the other, and obviously, they are of two different densities; hence we see the rays, or, in this case, the spoon bent at the point where the water or tea meets the air — the surface. In the case of the object lying on the bottom of the stream with ripples flowing over it, the object appears elongated or shortened by the ripples because of the curvatures presented by the surface of the wave forms. In the case of the "heat-waves" rising from the surface of the road distorting familiar objects, the air itself will be the only medium, but will vary in density as it becomes alternately hotter and cooler according to the "waves" of heat coming up from the pavement. Since hot air expands and cool air contracts, and as the air expands it occupies more space for the same actual amount of air, its density will decrease when it is heated and increase when cooled. It is this continuously changing density due to the "waves" of heat that will give the objects the appearance of fluttering, because the rays of light they are sending toward us are being bent by the changing density of the air. Basically, a lens works on the same general principle, although, strictly speaking, more like the second example given. Having a medium of constant density, glass, it is the curvature that it presents to a medium of second density, air, that brings about the bending of the rays. But here is where the similarity stops, because from here on everything is carefully planned to do a specific job. Instead of the rays being bent in any which way that nature might find them, they are bent according to a definite formula, so planned that they will come to a point, or converge, a short distance behind the lens. The rays of light coming from any ordinary object travel in straight lines — parallel lines. The lens bends these straight lines, brings them to a point, known as the focal point (or just plain focus), and the distance behind the lens at which these rays come to the point is known as the focal length, a very important measurement. The point at which the rays come to a focus is the point at which we will see clearly projected on to anything which we may wish to place in this particular position ( film, ground glass, paper, etc. ) , an image of whatever may happen to be in front of the lens. A lens of the type described is known as the simple meniscus lens. It is a single piece of glass with a convex curvature on both sides, or a double convex lens, and of the type generally found on box cameras, and is the simplest lens known. This lens has many defects, however, and is not generally useful for good photography because of these defects, or aberrations. A lens of the simple meniscus type will not bring into focus at the same point on the film all objects which are in a straight line the same distance away from the lens, or camera. In other words, if our camera were placed fifteen feet away from a group of people in a straight line, all of these people would not be in focus on the film at the same time, even though they are all exactly fifteen feet from the camera. This defect is known as spherical aberration. Secondly, lines that would be straight in the scene would appear to be curved on the picture (curvature of the field ) . Another defect that would be found would be that objects possessing the usual colors would not have the different colors come to a focus at the same point on the film — this defect is chromatic aberration. And still another difficulty would be astigmatism, or the inability of the lens to bring horizontal and vertical lines into focus at the same time. To correct these difficulties the modern "anastigmat" lens is actually a system of three or more lenses, or components, with the simple meniscus as its basis. In word, the solution is a simple one: algebraically, a plus two and a minus two equals zero. In the simple meniscus lens, the defects are measured as a definite positive quantity, then these same defects are ground in the opposite direction into another lens, or as a negative quantity. In practice, this "corrective unit" consists of two lenses, or components, one of them a positive lens, and the other a negative one, so that their dioptic power (their power as a lens) is zero also, leaving the simple meniscus lens in the rear of the system to do all the actual work of focusing the rays to a point. In this manner we have the simple lens working unmodified or otherwise changed by the system in front of it, yet with its defects or aberrations eliminated by cancelling them out with the front components. Scheibe's Hotspot Iris Projectionists who are employed in the transparency department find that they are bothered with the "hotspot", a flare of light on the screen that is the result of the arc in the center of the picture being hotter than on the sides of the picture. Many devices and methods have been tried to offset the "hotspot", but with the development of background projection effects a decade ago some cure was needed. I developed what is known as the "Hotspot In ris. Scheibe's Hotspot Iris is adjustable in many ways. It is used on the projector to eliminate the "hotspot" in the center of the screen so it will photograph as evenly as the sides of the screen. The Hotspot Iris is moved toward and away from the projection lens until the hotspot is eliminated from the screen. After the "hotspot" is eliminated the screen is photographed with the actors and actresse bet wen the camera and screen. Cameramen go out on location to photograph backgrounds for process work the world over and the Hotspot Iris aids materially in making such efforts possible. The Hotspot Iris is made in 6" x 6" and 8" x <>" with a blue or a neutral color in the center. About .50 neutral is the best color to use, though any desired color will he made. 22 International Photographer for January, 1941