How to Use a Digital Lux Meter for Flash Photography

Finding the correct settings for a camera is a mixture of art and science.

A lux meter is designed to measure the illumination in a location in a unit known as lux, or one lumen per square meter. It is the metric equivalent of the Imperial foot-candle. Photography, however, uses luminance as a standard of light measure, so the readings from a lux meter must be converted before use. Illumination is the amount of direct light from a source. Luminance is the amount of reflected light, which is typically measured from a gray card that has an 18 percent ink density.

Instructions

1. Take your lux measurement.

2. Convert lux to foot-candles, since most camera lighting charts use the Imperial standard. For precise conversion, multiply the lux value by 10.76, or estimate and just add a zero to the end. So, roughly, one lux would be 10 foot-candles. Then, convert to foot-Lamberts by multiplying by pi, or estimate by multiplying by 3. So one lux would give 30 foot-Lamberts of luminance.

3. Set your camera's flash according to the luminance that you want. Figure your total luminance which, for a bright, sunny day is typically 800 foot-Lamberts. Subtract the number of foot-Lamberts you figured from the lux meter, and set your flash to provide the difference.

Tips & Warnings

• The lux meter is not designed for photography. Photographic light meters will calculate exposure value, camera settings and flash settings automatically.

• Many modern cameras have built-in light metering, obviating the need for a lux meter.

• Experiment with the readings on the lux meter and the flash settings until you find what gives you the best results so that you can quickly estimate settings on the fly.

References

• John Lind: The Science of Exposure
• Cambridge in Colour: Camera Metering and Exposure
• Digital Photography School: The Light Meter