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Introduction to Light Meters
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Choosing Hand Held Exposure Meters
Hand-held exposure meters measure light falling onto a light-sensitive cell and converts it into a reading that enables the correct shutter speed and or lens aperture settings to be made. Hand-held exposure meters come in many variations, each with specific benefits. By using the appropriate meter for your specific needs, you can be assured of consistent professional results.
Incident vs. Reflected
The two distinct techniques of measuring light, incident and reflected, each have their own advantages in different situations. Hand held meters can give you both capabilities, along with features not found in even the most advanced cameras with built-in meters.
The 18% Neutral Gray Standard
Light meters are designed to measure light in a consistent way. They cannot see the subject and interpret it as a photographer can. For example, a light meter cannot distinguish a black cat from a white cat, a red balloon from a blue balloon, nor textured powdery white snow from a shiny white auto paint finish. Given the same lighting situation, each of these objects would reflect a different amount of light.
Reflected measurements would indicate different exposures for each object. Incident measurements would indicate the same exposure for each object, to render a consistent exposure. Light meters are calibrated to assume that all subjects are of average 18% reflectance, or neutral gray. The use of the 18% neutral gray standard allows a reflected light meter to render correct readings for “average” subjects in “average” lighting situations. (The value of 18% neutral gray is also referred to as Zone V in the Zone System, an advanced black and white exposure method.)
The incident meter is aimed at the light source and measures the light source falling directly on a scene and is not influenced by the reflectance of the subject being photographed. For more precise control of the photograph, incident meters are also used to measure various levels of light from multiple sources falling on separate parts of a scene.
Using Incident Meters
Incident metering measures the intensity of light falling on the subject and gives accurate and consistent rendition of the tonality and contrast regardless of reflectance, background, color, and shape. Subjects that appear lighter than gray will appear lighter. Subjects that are darker than gray will appear darker. Colors will be rendered accurately. Highlight and shadow areas will fall naturally into place.
NOTE: Most light meters allow for both reflected and incident light readings.
Advantages of Incident Measurement
Incident meters measure accurately and consistently and are not affected by variances in reflectance of the subject or scene. Because of this, incident meters give the most accurate exposure for the majority of situations and subjects.
Reflected metering reads the intensity of light reflecting off the subject and may vary according to variances in tonality, color, contrast, background, surface, or shape. Meters are designed to regard all subjects as 18° neutral gray reflectance. Reflected measurement of any single toned area will result in a neutral gray rendition. Subjects that appear lighter than gray will reflect more light and result in an exposure that renders it darker. Subjects that are darker than gray will reflect less light and result in an exposure that renders it lighter.
Using Reflected Measurement
Hand-held reflected light meters and built-in camera meters read the intensity of light reflecting off the subject and measurements are taken from the camera position. Generally, reflected measurement of a wide subject area can include many different reflective surfaces or colors that can bias the meter and result in inconsistent and erroneous readings. Accuracy of a reflected measurement can be improved by reading an 18° neutral gray test card placed in front of the subject.
Illustrations & copy adapted courtesy of Mamiya America Corp.
Sekonic Professional Division. Total Exposure Control with Hand-Held Metering
Incident vs. Reflected.