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AMATEUR MOVIE MAKING 41
Exposure Meters. — The exposure meter, so-called, may be a set of reference tables, a calculator constructed something like a slide-rule or it may be an actual meter which measures the intensity of the light falling from the sky or the intensity of the light reflected from the object which is to be photographed. It is obvious that both reference tables and calculating slide rules will give only average results, but as most of these calculators have been designed after a study of hundreds of test exposures, it may be assumed that the results represent the mean values of all of these tests. So nearly identical is the light intensity under certain given conditions that these calculators are quite satisfactory and thousands are in everyday use. They require a little time to operate and are not as convenient as some other kinds of meter, but their low cost makes them very popular.
The Harvey Motion Picture Meter is typical of the calculators. In this meter we have two sliding scales. We start by setting a pointer to indicate the correct geographical location and time of year. Setting this pointer changes the position of one scale. We then set a second scale adjacent to the first. The second scale carries a list of light conditions. The adjacent edge of the first scale carries the frequency factor, such as normal, one-half normal speed and so forth. It also carries indices indicating the height of the sun above the horizon, for use late in the evening or early in the morning. When these two settings have been made the meter is turned over. Here we find a series of "windows" arranged in horizontal and vertical rows. Each entire vertical row represents one diaphragm (lens) stop, ranging from f 1.9 to f 32. The horizontal rows indicate the subject, each row being given to one typical group of subjects. There are 10 diaphragm stops and eight classes of subject giving us a range of eighty readings. When the slides have been set we find that most of these windows are blank, but in some of them we see small dark spots which represent, respectively, sectors of discs equal to 180 degrees, 135 degrees and 90 degrees, the three most common settings for adjustable shutters. If we find that for a beach scene the 180 degree sector appears under 16,