Movie Makers (Jan-Dec 1952)

171 A CLOSEUP COMPUTER For titles — and all other close camera work — this easy-to-make slide rule computes distance, diopter and field coverage THE RULE IN USE HOWARD A. MOORE, ACL THAT was, I thought, a pretty convincing case for the diopter lens which Herbert D. Shumway, ACL, made in April Movie Makers with the second installment of his two-part Let's Make A Nature Movie. (By editorial design, the present author also contributed to the overall presentation with What Diopter Lenses Do — Ed.) PROBLEMS OF DIOPTER FILMING As adapted to nature filming specifically, Mr. Shumway's system of focal frames did away with the major problems of supplementary lens shooting. ' These are, of course, what strength of diopter lens to use at a given distance (or, conversely, at what distance to film with a diopter lens of a given strength) , and what field area will be covered by the camera at any given combination of distance and diopter. With the focal frame system, these queries are automatically answered by the frames themselves— once they are accurately designed and positioned. There are other filming operations, however, where the focal frame is not the handiest technical setup for the subject involved. Title making, for example. Here also the supplementary, or portrait, lens is a tremendous boon to the beginner's camera. But without a focal-frame setup, its use requires reference to one table or another of diopter strength ratings, field areas and camerato-subject distances. Hoping to simplify this research operation, I have combined all of these values in a handy diopter slide rule. DESIGN OF SLIDE RULE You will find its component parts plotted on the opposite page. These are the top piece A, the slide piece B, a bottom piece (undiagramed, since it is simply a plain sheet of cardboard similar in size to the top piece), and four spacer strips suggested in the Slide Rule Assembly sketch on this page. As with a previous slide rule design (see Exposure By Slide Rule, September, 1951 — Ed.), the essential parts (A and B) are reproduced in equal scale, so that they may be cut out from the page and applied to the cardboard skeleton of the rule itself. ASSEMBLY OF SLIDE RULE For a suitable cardboard, I used a red pressboard which is a durable ma TOP PIECE A SLIDE PIECE "b"~ I / \ BOTTOM PIECE'' / ""--SPACER STRIPS"" SLIDE RULE ASSEMBLY terial that lends itself well to being cut with sharp, smooth edges. On it I mounted separately the outlines of the top piece and the slide piece, using photo tissue as an adhesive (although rubber cement will do). Then, with a straightedge and a sharp knife, I cut the pressboard along the exact outlines of the top and slide pieces and then cut out the windows where they were indicated on the top piece. A bottom piece (similar in size to the top) and four spacer strips equal in length to the assembly should now be cut out. These latter are used in pairs to separate the top and bottom units, thus assuring ample room for free movement of the slide. In assembly, the top, bottom and spacer units are simply cemented together (as shown in the slide-rule-assembly design) , with the slide unit running freely between them. With my own rule, I then drove in four staples along each edge for extra strength, and, to make the slide free-wheeling, I sprayed it with a coat of transparent plastic. DIOPTER, DISTANCE, AREA What, now, are the values which this handy little gadget will determine for the user of diopter lenses? An analysis of its face plate will answer that question easily. Reading from top to bottom, we find windows showing the diopter rating of the supplementary lens in use ; the lensto-object distance in inches; the field area (width and height) covered by a 25mm. lens on a 16mm. camera, and field area covered 'by a 12.5mm. lens on an 8mm. camera. There is also a further refinement in the field area readings. For the circled figures indicate the actual area recorded on film by the camera; the plain figures, on the other hand, indicate only how much of that area the projector will transmit as a screen image. This latter area, of course, is always slightly smaller than the area pictured on the film itself. For a simple example of the slide rule in use, let us suppose that you have available only a plus-2 diopter lens. In making a title with it, you will want to know (1) what the camera-tocard distance should be, and (2) what the field area or coverage will be with, say, the V2 inch lens on your 8mm. camera. To determine these values, you would proceed as follows . . . Align the slide in the top window (diopter of supplementary lens) so that the figure 2 will be under the arrow. Now in the distance window you will find that the arrow indicates a camerato-card distance of between 19 and 20 inches. For a more exact reading, refer now to the fine lines at the top of the window, which are calibrated in % of an inch. Here we find the arrow pointing at V2 an inch, for a combined distance reading of 19V2 inches. In similar fashion, we may determine field coverage by referring to the bottom window — for 8mm. cameras. Here, in the circled numbers, we find that the camera field recorded on film will be 5% by 7% inches, but that the projected image field will be reduced to 5Vs by 6% inches. STILL OTHER USES However, reading from top to bottom on the rule is only one of the many ways in which it may be used — and not always the most helpful. Let us now suppose that you have an outstanding still picture in full color which you wish to use as a title background. Upon measuring it with a ruler, you find that Its overall dimensions are a bit more than 9 by 12 inches. These figures now become the known values, and by setting them on the rule we can work backward to determine both distance and diopter lens required for such title shooting. Using the plain (projector image) 9 and 12 figures on the 8mm. scale, we find specifically that the camera-to-card distance is 35 inches, the suitable diopter lens a plus-1. Still another possibility will assume that your only titling setup of the moment restricts you to working at a camera-to-card distance of 24 inches. Setting this known value on the distance scale, we find that the camera coverage (or minimum size of card needed) will be 61/2 by 9 inches, while the diopter lens needed should be a plus-1 %. In such an instance, a plus-2 lens would probably serve as well. These, then, are some of many ways in which this diopter slide rule may aid you. And, bear in mind, it will be just as handy in plotting all extreme closeup shooting. . . . You'll probably want to put your rule together tonight.