American cinematographer (Oct 1933)

October 1933 • American Cinematographer 215 V-1 SLUE Above—Relative values as interpreted by the naked eye; Below—relative values as interpreted on Panchromatic film. Special Effect Use of Filters Part IV by Hartley Harrison Editor’s Note: This is the fourth in the series of articles on this subject by Hartley Harrison, well- known manufacturer of color and effect filters . W E HAVE seen from the discussion in the previous article that the manner in which we wish to control or change the relative contrast of the various colored objects in the scene determines broadly the density of the filter. However in determining the exact density necessary to give the change'of relative contrast that is wanted, some filter measuring device is highly desirable. It eliminates to a large degree the guess work in the final selection of the filter. One of the simplest means of doing this is to use a panchromatic viewing glass that will compensate for the difference between the color sensitivity of a normal eye and that of the film that is being used. As these are a great many types of viewing glasses we will explain the type that is referred to as panchromatic viewing glass and explain its application relative to measur- ing color filter densities. Also let me add that the panchromatic type of viewing glass has been on the market for several years and is made by several different manufacturers. There has been a great mass of information written on the subject of the difference between the color sensitivity of a normal eye and panchromatic films, but we will only touch on the high lights of these differences as they relate to measuring the exact density of a color filter necessary to create the desired change of relative contrast when using a viewing glass. As the color of the various objects in the scene deter- mines the amount of light, within certain general limits, that is being reflected from the objects, and as the film does not have the same sensitivity for each different color, the viewing glass, in order to allow a person to see the relative contrast of the various colors must, first transmit all of the colors, second, transmit the colors in the pro- portions to which the film is sensitive to the various colors, and third compensate for those colors to which the eye is more sensitive. Or in other words if we photograph wedges of blue, green; yellow and red with panchromatic film as shown in Figure 1, and each reflects the same amount of light, only of a different color, we will find that the blue wedge is the lightest, the green the darkest, and the red and yellow have a density between these two. But looking at the colors visually the yellow will be the lightest, the blue the darkest and the red and green will have a density that is in between the two, as shown in Fig. 2. Therefore the viewing glass when viewing the same colors through it should compensate for those differences and make the blue appear the lightest, the green appear the darkest and the red and yellow in between, exactly the same as the film will photograph the colors. By viewing a scene through a viewing glass that will give this compensation we can more accurately judge how the relative contrast should be changed to create the desired effect. Then by adding a color filter to the viewing glass so as to look through both at the same time, the change of relative color contrast that is seen through the combination will be had when the color filter above is used to photograph through. Because the viewing glass has in effect changed the color sensitivity of the eye to the same sensitivity of the film by adding the color filter to the viewing glass and looking through it, it is substantially the same as adding the filter to the camera and photographing through it. If compensation for the difference between the visual balance and the photographic balance is all that is desired when a filter is used, the test is simple. We first determine with the naked eye just what that balance is in the scene. Then we add in front of the view- ing glass a filter that will give us the same balance as we secured with the eye. Just what that filter is to be, of course is determined by the colors being photographed. By this method we secure the photographic value of the scene when that particular filter is used. The use of the viewing glass alone, of course gives us the value without a filter. Having that base to work upon, it of course becomes reasonably simple to determine what filter is to be used, by determining the color contrasts we want and then applying the filter in front of the viewing glass to get a more definite interpretation before we expose the film.