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PAGE 234
HOME MOVIES FOR MAY
Authored by Wm. Lee Morgan
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• Pictured here ore the three most common types of photographic exposure guides and their method of use — The photoelectric exposure meter; the exposure scale or "chart" and the "extinction type meter." The photoelectric meter is most accurate under adverse light conditions.
J. HERE are three kinds of exposure guides in general use: (1) the chart or scale; (2) the extinction type exposure meter; (3) the photoelectric type of meter.
The exposure chart is the simplest of all. It is a list of the various lighting conditions you will encounter. Opposite these are the “f” stops you will use at 1/50 of a second. This is usually the only shutter speed mentioned in the chart, as if you wish to shoot at another speed, you can stop down one full stop if you double the shutter speed (1/50 doubled is 1/25), and open up a stop each time you cut the shutter speed in half. Charts of this kind are usually found in the film you buy, and refer only to that particular film or other films of the same rating.
The exposure scale may be either a circular disc mounted on a card, or it can be the slide-rule type. The user has to judge which lighting condition listed on the scale best describes the conditions under which he is making the particular exposure. For example, he might select: day-cloudy bright; subject back-lighted. From this it is a simple matter to set dial or slide-rule and find the “f” value opposite the shutter speed you want, or vice versa.
The extinction type of meter measures the intensity of light reflected from the scene to be photographed. It usually consists of a small tube which is held to the eye, and the subject viewed through it. Inside the tube is a small translucent object which transmits luminosity in a graded field. As the user looks through the tube, he sees numbers across this field, becoming dimmer as they become greater in numerical value. The biggest number he can see is the key number to the
exposure for that scene. Most meters of this type have a calculating scale right on the tube, which is adjusted to determine what stops and shutter speeds to use together. Usually there is also an adjustment which takes care of film speed rating.
Extinction meters are valuable in that they take a critical reading of light intensity, eliminating guessing. It should be remembered, however, that the human eye does not always see objects the same each tme. For example, if you had just come out of a darkened interior, as a motion picture theater, you would obtain an entirely different reading from that made if you had been out in the sunlight for awhile.
The photoelectric type of exposure meter, as its name implies, employs a light-sensitive photoelectric cell which changes light into electrical energy, activating a moving pointer. The pointer, against a scale of figures, indicates the light reading, which is used as an index on the calculating dial. Shutter speeds are then opposite the correct lens opening.
Some meters give direct readings, the sensitive pointer indicating the lens opening to use in that light. However, this is applicable only to a given shutter speed and film rating. For other speeds and ratings, it is necessary to figure on the diaphragm scale, either up or down, to determine exposure.
A LL film has a definite sensitivity, called film speed rating. As there is considerable variation in this rating between different types of film, it is necessary to pre-set the meter for the rating of the film you are using. The Weston meter and several other American-made meters use the Weston system of rating film speeds. As an example of the ratings used, a film for general outdoor use would be rated at about 24 Weston. It might be valuable to know that Weston ratings are directly proportional to film sensivitiy, a film with a rating of 12 Weston being half as fast as a film of 24 Weston, and thus requiring one stop larger opening or double the shutter speed.
• Continued on Page 250