How light meters work

Mark M. Hancock / © The Dallas Morning News

Chris (left) and Nancy Tanner (right) of Denton kiss at Medici, a private lounge for Nick & Sam's customers, in Dallas on Friday, May 21, 2004.

A light meter uses a photocell to measure the amount of light falling upon a subject (ambient light) or reflected from a subject (reflective light). Almost all modern cameras are equipped with some form of light meter (usually reflective).

A perfectly focused view of a world-changing event is worthless if the exposure is drastically wrong. PJs must know when the meter is wrong and compensate. In other posts we discuss the Sunny 16 Rule and dynamic range, but let's take the little steps first.

Reflective light meters
For consumer uses, these meters often set the correct exposure for most general subjects. Some images may be over or under exposed, but 10 bad frames from the vacation is an acceptable loss for most consumer camera operators.

The reflective light meters in most cameras see the world as neutral gray and make an averaged reading. A typical meter reading turns a white wall gray (underexposed) or a black tire gray (overexposed). The key to photography is allowing the right amount of light to hit the film or digital sensor.

Pro reflective light meters
Modern high-end professional cameras measure reflected light and compare the information collected from an array of sensors through a complex algorithm of similar images to produce the most probable subject and appropriate exposure. It isn't as accurate as an ambient light meter, but it's better than a standard reflective light meter.

For instance, a modern professional camera understands the PJ is about to photograph a full moon on a cloudless night and gives a reasonably accurate reading compared to a standard reflective meter, which would produce a drastically wrong reading.

For those interested, the light reflecting from the moon is the same as it is during daylight on Earth because the moon's distance from the sun (the light source) is similar to the Earth's distance from the sun. This holds true with the general inverse square law. We'll talk more about this one day.

Ambient light meters
Ambient light meters are often more accurate than reflective light meters. Ambient light meters measure the actual light falling on the subject. It doesn't matter if the subject is black velvet or a polished silver tray. The light reading is stable with a constant light source.

If PJs are unable to meter the light near the subject (during a hostage standoff for example), an ambient light meter in the same light source, direction and distance yields a correct exposure. It's important to stress the light must be from the same source, direction and distance (inverse square law of light). If artificial light is the source, it's very possible to get a wrong reading from disparate locations.

Ambient spot meters
For a higher level of accuracy, a spot cover can be placed over a hand-held, ambient light meter's photocell. It limits and measures light from different directions to give the PJ more control over which light source is most accurate for the subject.

A dedicated, incidence spot meter is the most accurate of all meters. It measures the exact amount of light falling on the photocell from a limited angle of light. Often, these meters also measure flash output, color temperture and more. They also come with an appropriate price tag.

Enough for now,