EXPOSURE MEASUREMENT-continued
Exposure Control
The amount of light reaching the film plane is deter- mined by a combination of the lens aperture and the shutter speed. Since the two are interrelated, different combinations will give the same exposure. A 1-step change in the shutter speed, or a 1-stop change in the aperture setting, will either halve or double the exposure. For example, a shutter speed of 1/125 second passes twice as much light as a setting of 1/250 second, and only half as much light as a speed of 1/60 second; for an aperture setting of f/11, twice as much light as f/16, and half as much as f/8, is passed. This feature characterizes the oJ)eration throughout the available range of shutter speeds and aperture settings. With this in mind, it's easy to see that if a correct exposure for a scene is 1/125 at fill, then 1/60 at f/16 or 1/250 at f/8 will be equally acceptable.
The best comb ination for your needs will depend on the results desired. Use fast shutter speeds to freeze motion, or use slow speeds to produce deliberate and creative blur. Small apertures give greater depth of field, while large apertures restrict sharp focus to the main subject. The creative selection of both speeds and apertures will greatly enhance your photography.
Metering Range
If the center "correct exposure" LED fails to illuminate, even after all possible lens-aperture/shutter-speed com- binations have been tried, then the available light is too bright or too dim for the meter's range. To correct this situation, several measures may be taken, as follows: Switch to a new film (either higher or lower ASA) that more closely matches the available light; mount a neutral density filter on the lens to decrease the light reaching the film plane; or use artificial lighting (i.e., an electronic flash unit) to increase subject illumination. Remember, too, that the lens in use can greatly influence suitability for bright or dim shooting. For example, a 50mm f/1.4 lens (with ASA 100 film) couples from EV - 2 (f/1.4 at 8 seconds) to EV 17 (f/8 at 1/2000 second) for excel - lent low-light performance; on the other hand, a 200mm f/4 lens proves more usable at bright-light levels, coupl- ing (with ASA 100 film) from EV 1 (f/4 at 8 seconds) to EV 20 (f/22 at 1/2000). Thus, choose the lens carefully to match the existing lighting conditions.
