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44 AMATEUR MOVIE MAKING
and 180. Now if we refer to the right side of the dial we see the usual diaphragm stops from f 1.9 to f 32, while on the right side of the rim of the case we see some figures marked "EXP." These figures refer to the exposure frequency, the actual values indicating the number of frames exposed per second. (Remember that normal speed is sixteen frames per second.) We find 16 on this scale and opposite it on the dial we see f 1.9. This indicates that we can only make this exposure with an f 1.9 lens. However, if we have a 2.5 or 2.7 lens we can run the camera at half speed and still get the film, for we see 2.5 opposite 8, which means 8 frames per second. This meter has proven very popular, but with it we are again measuring the light which falls upon the subject. At first glance this might appear to be the logical procedure, but the fallacy will be apparent upon consideration.
We have seen that light falling upon the sensitive film causes the photographic action to take place. The fact that this light, which does so affect the film, is reflected from the subject makes possible the formation of the image of the subject. Therefore, it is evident that the light with which we are working is solely the light reflected from those objects whose images will appear in the completed photograph. Now if we have light of equal intensity falling upon a piece of white cloth and upon black velvet, we have a great difference in the amount of reflected light. This makes the difference in the appearances of the cloths. Objects which are photographed are usually composed of a great^ number of shades of dark and light, and the general tone makes up a light, medium or dark object. It is obvious that an exposure calculated for the reflected rather than for the incident light will be the more accurate. For measuring the reflected light we use the extinction meter.
The extinction meters consist, usually, of round cases in which a disc revolves. This disc is quite transparent at one place and then gradually darkens until it becomes practically opaque. This opaque portion joins the transparent portion. This tinted disc or "optical wedge" is usually tinted blue. Now, upon looking through a peep hole provided for the purpose we see the scene before us