Movie Makers (Jan-Dec 1952)

107 WHAT DIOPTER LENSES DO A knowledge of Why, as well as How, makes clear the operation of these accessories HOWARD A. MOORE, ACL IN the entire field of amateur filming I doubt if there is any other accessory which so enlarges the scope of a simple camera as does the supplementary lens. And yet, in hobby-chatting with other amateurs, I find that there are few who understand them. Perhaps this confusion stems from the several differing names used to describe these wonder workers. For you will hear them called supplementary, accessory, portrait or diopter lenses. And, in the Kodak trade name, they are (quite properly) known also as Portra There are, in many an operation connected with movie making, two important phases of knowledge concerning it. The first phase is the practical one, expressed in the word How. The second phase is the theoretical one, expressed by Why. In Let's Make A Nature Movie, the article just preceding this one, Movie Makers presents the "How" of using supplementary lenses in extreme closeup filming. In the discussion on this page we present the quite-simple "Why" of these effective accessories. With them, even the simplest camera takes on new powers. CAMERA LENS PARALLEL LIGHT RAYS FROM A POINT OF LIGHT/ AT INFINITY---- ' (SEE TEXT ) \ (FOCAL LENGTH J A* DIAGRAM "A" FOCUS ON FILM '■FILM > (FOCAL LENGTH) LENS--' DIAGRAM "B" T '^RAYS FROM A POINT OF LIGHT LEAVE LENS IN PARALLEL LIGHT RAYS > a >s'L (FOCAL LENGTH ) (SEE TEXT ) (FOCAL LENGTH ) FILM DIAGRAM "C" THE OPTICAL ACTION of a supplementary lens is made clear above. Diagram A shows the converging or focusing action of a camera lens. Diagram B plots the ability of a diopter lens to turn diverging rays into parallel. In diagram C we see the result of combining the two lenses. lenses. But call them what you will, they are a boon to every amateur using a fixed focus camera — and even, on occasion, to those movie makers equipped with focusing lenses. In any discussion of these accessories you will come early upon the word "diopter." This is as it should be, for a diopter is simply a unit of measurement— in this case a unit used by oculists to express the power of a lens. A supplementary lens of 1 diopter strength is one which has a focal length of one meter, or 39.37 inches. Used in conjunction with a fixed focus lens (or a focusing lens set at infinity), it will create sharp focus on subjects at a distance from the camera equal to its focal length. In other words, roughly 3 feet. A lens of more than 1 diopter strength will have a focal length equal to 39.37 divided by its diopter rating. Thus a 2 diopter lens will have a focal length of 39.37/2, or 19.68 inches; a 3 diopter lens a focal length of 13.12 inches, etc. In the photo field these lenses are often referred to as plus 1, plus 2 and plus 3 portrait (or Portra) lenses. To understand why these supplementary lenses operate as they do, let us examine first how the primary lens in your camera operates. In diagram A we see parallel rays of light coming from a point of light at infinity, the rays being parallel because, in effect, the point source is so far away. We see also how the camera lens converges these rays so that they intersect in a point image on the film. The distance X on the diagram is the focal length of that camera lens. Now look at diagram B. Here we see another lens which reverses the process. From a point source it accepts diverging light rays and passes them on as parallel rays. The point source is at the distance Y from the lens, which distance becomes the focal length of that lens. Very well. This immediately suggests that if we combine these two units, lens B can pass on parallel light rays to lens A which, in turn, can then focus these rays on the film. And, as will be seen in diagram C, this is exactly what happens when we place a supplementary lens in close contact with a fixed focus camera lens. Distance Z, the negligible space between the two units, is so slight that it may be disregarded. Distance Y becomes the new focal length of the combination, and objects at that distance will automatically be in sharp focus.