American cinematographer (Jan-Dec 1942)

Frame enlargement of scene made with cylinder distortion. DIDPTERS-FDH DISTURTIDIV! By JOSEPH WALKER, A.S.C a D lOPTER" lenses — the inexpensive spectacle-lenses that can be picked up at the five-and-ten or at any spectacle-making shop — are familiar to most amateur cinematographers. They're useful as supplementary lenses to correct the focus of camera lenses for extreme close-up work, such as making titles or extreme full-screen close-ups of flowers and insects. But they've another use, too: in the proper combination, they can be used to produce deliberately distorted optical effects on the screen. This distortion can be either small or great, as the scene demands. It can be just a slight compression of a scene in one plane (either vertical or horizontal) with an accompanying spread in the other plane, in order to make a landscape composition fill the frame more pleasingly, or to give a close-up something of the lengthened perspective you see in fashion sketches and some 'of William Mortensen's still pictures. In this case, the distortion would probably pass unnoticed. Or it can be exaggerated for special effects, such as making a Boris Karloff-like character more menacing, or adding either me:iace or grotesquerie to a nightmare sequence. First of all, though, let's get started with a good understanding of what a "diopter" lens really is, and what it does. "Diopter" is simply the tei-m used by opticians to designate the magnifying power of a spectacle-lens. One diopter is the reciprocal of a focal length of one meter; thus a 10 diopter (10 D) lens would have a focal length of 1/10 meter or 100mm. Diopter lenses are available both as positive lenses (rated as phis so many D) and negative lenses, rated as minus so many D. Objective Eye-piece QflLIL[HN TELESCOPE SYSTEM. For distortion, we make use of a somewhat more complicated supplementary lens set-up, using two cylindrical diopter lenses — one positive, and one negative — assembled like a simple Galilean telescope, as shown in the sketch. A plus diopter cylinder occupies the objective l)osition, while a minus cylinder of greater power takes the place of the telescope eyepiece, and is placed as close as possible to the camera's lens. Both the axes and the focal points of the two cylinder lenses must coincide. The separation between the two should be equal to the difference in their focal lengths. The magnification produced may be determined by dividing the focal length of the plus lens by that of the minus lens. From these facts, it is easy to calculate any type of system you want. Suppose you use a -)-10D cylinder for your plus lens, and a -20 D cylinder for your minus lens. The plus cylinder will have a focal length of 100mm; the minus cylinder will have a focal length of .50mm. The separation between the two should equal the focal length of the difference in their focal lengths, that is, 100mm. — .50mm., which equals .50mm. The magnification or distortion produced is equal to the focal length of the plus lens divided by that of the minus lens, in this case 100-^50, which equals 2. Mounting a supplementary lens system like this presents some difficulty these days, since the tube that holds them should keep them accurately aligned, and yet permit adjusting the separation between the two, and also permit one of the lenses to be rotated with respect to the other, so that the final adjustments can be made with the device in position on the camera. Ordinarily, a pair of metal tubes, one of which fits fairly snugly inside the other, would be the thing to use. But today, most metals are on priorities, and most machine-shops (other than home workshops) are likely to be busy on more important jobs than gadget-building for amateurs. Luckily, though, spectacle-lenses are so light that you can get around priorities by using stiff cardboard tubing of appropriate diameter. This incidentally has the advantage of being light enough so the gadget can be mounted directly on the camera lens, rather than requiring a supporting arm extending from the tripod-head. In a mount like this, you can hold the spectacle-lenses in place either by using cardboard retaining-rings, which can be glued into place, or Scotch tape. For long-shots, this system is complete in itself. But if you are going to use it making closer shots, you'll need to add a spherical auxiliary lens {not a cylinder lens as in the distorting system) in front of the plus cylinder to correct your focus. The focal length of this supplement should be equal to the distance between camera and subject: that is, if the subject is 6^-2 feet from the camera, use an auxiliary with a focal length of 2 meters — or a plus 0..5() D spectacle-lens — to refocus your optical system at fi^L' feet. The amount of distortion is determined by the overall power of the supplementary-lens system: for most purposes a set of these cylinder-distortion supplementaries ranging in magnifying power from 1.2 to l.fi will prove satisfactory. More ]iowerful units can be (Continued on Page 370) 358 August, 1942 American Cinematographer