Understanding Light Meters
October 3, 2014
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Photography is the art and science of capturing light, so it's no surprise that a lot of technology is dedicated to measuring the amount of light falling on your subject. Understanding how light meters work and how to use them is key to capturing properly exposed images, whether you use the meter built into your camera or a handheld model.
Principles of Light Metering
There are two basic types of light meters: reflected and incident meters. Reflected meters measure the amount of light being reflected back from the subject. This makes for a convenient way to measure light because you don't have to be physically near your subject to measure the light reflecting off of it; however, highly reflective subjects can skew meter readings dramatically. All in-camera meters are reflective meters.
Incident light meters measure the amount of light falling on the subject as opposed to being reflected from it. This allows for more accurate readings but requires you to be able to physically position the meter in front of the subject, facing the camera for the most accurate reading. Incident meters are found in handheld light meters.
In-Camera Metering Patterns
Spot metering measures a very small portion of the scene — between one and five percent, typically at the center of the viewfinder. Some cameras will allow you to use a selected autofocus point as the spot-metering location, however. It's most effective in high-contrast situations such as backlit subjects.
Center-weighted average metering concentrates 60 to 80 percent of the metering weight to the center of the frame, with the logic that most subjects are centered within the frame. The remainder of the metering weight slowly fades out to the edges, paying very little attention to the edges of the frame, which often are disproportionately contrasty, as with bright noontime skies. Center-weighted metering is good for general shooting situations and any time the subject is centered in the frame.
Average metering considers the light from the entire scene inside the viewfinder, then takes an average exposure based on all the measured points. No portion of the scene carries any more weight than any other. This is often an excellent choice for low-contrast scenes or landscapes where you are trying to achieve an acceptable contrast ratio across the whole image.
The meter built into the camera is perfectly acceptable for most lighting situations you will encounter in the field, but for studio work, you may find a handheld meter more useful, especially when working with studio strobes. Camera meters are not capable of measuring the output from non-TTL strobes, such as most studio lighting. On the other hand, most modern handheld meters can measure the output of studio strobes and give you the correct exposure for your subject and the amount of light output.
It's important to remember that light meters are calibrated to render a correct exposure for a subject that is middle gray — a shade of gray halfway between pure black and pure white, reflecting 18 percent of visible light. Healthy green grass is a good practical example of a middle-gray subject. Subjects that are significantly lighter or darker than middle gray will require some exposure compensation to ensure correct exposure.
Like all of photography, knowledge is only the first step toward using your light meter correctly. Each meter, whether in-camera or handheld, may read slightly differently depending on the situation, the lighting and the subject. Learning how your particular meter works comes with experience, so practice will make perfect — exposures, that is.
Photo credit: morgueFile