American cinematographer (Sept 1935)

374 American Cinem.atographer • September 1935 T he new Eastman "Pola-Screens” differ from all ofher screens in that there is no element of uncertainty connected with their use. Visual inspection of the image on the ground gloss shows precisely the effect that will carry through to the final print. Tricky variations of lighting and atmosphere, color-transmission, color-sensitiv- ity, laboratory treatment, and so on, which con of times deceive even the most experienced technician in matters of filtering, hove nothing to do with the action of the polo- screen: if you see the desired result on the ground gloss, you will see it in the negative. For the benefit of those who hove not read the excel- lent article explaining these screens, which appeared in the lost issue of this journal, it may be repeated here that polo-screens ore not filters in any sense of the word, but screens which, rightly manipulated, con cut out the plone- polorized light rays which produce glare, reflections, etc., while hoving no effect upon the diffuse, un-polorized rays which we use to form our picfure. Th.s action can best be understood by the following rough simile: if you focus o lamp (such os o standard ”18," for insfonce) upon on object or surface, you often get o strong glory reflection from that surface; in some instances, os for ex- ample polished melol surfaces, you get on image of the lamp itself. If you place o diffusing screen on the lamp, these reflections and the glare ore lessened. In the first instance, you get the glare because o great percentage of the light-rays ore parallel, vibrating in essentially the some plane; striking the surface together, they tend to reflect together. In the second instance, the diffuser has some- what scattered the rays; striking from innumerable angles, they reflect diffusely, so that we see the object itself, rather than a glaring reflection of the light. Often in nature these two types of light rays travel together, and to get the best picture, we should in some way separate the glare-producing rays from the diffuse rays which we want to make our picture. That is what the pola-screen does. When rotated to the proper angle, it stops the parallel, glare-producing rays, but lets the scattered ones which form the image of the object go through unchanged. This is why the action of the pola-screen can be determined visually. The use of these screens is wholly a matter af angles. With the lens pointed at the object at a certain angle, the pola-screen will give the maximum result; at other angles, the glare-eliminating action is increasingly less. In the same way, the pola-screen itself must be rotated to the angle at which it most completely stops the glare- components of the light; at other angles, its action is pro- gressively less, until at right angles (90°) to the optimutr. position, it actually increases the glare in many instances. What are these angles? For eliminating reflections, glare on water, and so on, the camera should be shooting the object at an angle of 32°. For eliminating the glare from "hot” skies, and for getting darkened skies such as we get with a red filter, but without changing color-rendi- tion of other areas, the lens should be pointed at an angle of 90° away from fhe sun: in morning or evening, either north or south; at noon, around the horizon in any direc- tion. The position of the screen itself must always be found by visual inspection; this is very simple, for half a turn of the screen will instantly reveal at which position it is most effective. Obviously, then, the pola-screen is not one of those devices that can be put on a camera and virtually for- gotten. Far from it: it is very decidedly a tool for special Shooting through store window, note reflections without pola-screen. Practical occasions. Either you need it badly—or you don't need it at all. If you don't need it, while it can hardly do any harm if left on the camera, it will serve no useful purpose, and will eat up quite a bit of light, for its multiplying factor is 4. For this latter reason, too, it would seem that the pola- screen will find its greatest usefulness in exterior photog- raphy. On the stages, where the Cinematographer has his lighting under perfect control, there should be less need for the pola-screen's glare-eliminating ability—and the fact that the screens demand two full stops increase in ex- posure is definitely a disadvantage when filming interiors. The question has sometimes been asked, "is it not possible to get the same general results by using neutral density filters, rather than the polo-screens?" The answer to this is a decided negative. The Neutral Density filter (even the graduated filters of this type) acts over the whole area of the picture. The pola-screen, on the other hand, is definitely selective. Where, with a pola-screen, you can reduce or eliminate glare from any given area, without in any way altering any other part of the field, with a Neutral, you "tone down" not only the glare, but every other part of the picture as well. In the comparatively short time since pola-screens were made available in Hollywood, a number of studios and indi-