Basics
Photography is the capture of light and shadow. How much light and how much shadow are what determine how a photo will be exposed. To gauge this, photographers use light meters, either built into their cameras or as hand-held devices. The light meter determines how much light is in a scene and, through dials or digital readout, tells photographers what shutter speed and what aperture setting should be used with a particular "speed" of film. Film speeds generally range from 25 to 3,200 ISO. The smaller the number, the slower the film, meaning more light and slower shutter speeds are needed to expose it. This film speed idea has carried over into digital cameras, most of which have ISO settings. ISO is the equivalent of the former ASA number and replaced the American ASA and European DIN.
Inside the Meter
Light meters have a light sensor, usually made from silicon, CdS (cadmium sulfide) or selenium. Silicon and CdS both require power to operate, usually a battery. Selenium does not because it produces power enough to move a needle on the meter. The other two, of which most modern light meters are made, need a boost to move the needle that comes from the electric current of the battery.
What the Meter Does
Analog light meters have a dial or two and a row of exposure numbers with a needle. You set the film (or digital) ISO speed on a dial. The photocells are exposed to the light of a scene causing the needle to swing to a certain point. You rotate the exposure dial to match the needle position. This will tell you what to set the shutter speed and aperture opening to. Digital meters work essentially the same way except they have digital readouts that tell you the ideal exposure setting for your light conditions. In-camera light meters use reflected light, meaning the light from the overall scene. Hand-held meters also can use reflected light, but they can be used to gauge incidental light by being held in front of the subject to be photographed. Incidental light meters have a distinctive white ball and give more precise image exposure readings.
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