Exposure measurement is used in film and digital photography to measure light and suggest parameters that will provide a properly exposed image. Over time, the light cells within these counters may lose sensitivity and cause erroneous readings. Counters can be calibrated, or measured for accuracy, against a centuries-old principle of photography known as the Sunny 16 Rule. When opening f / 16 on a sunny and cloudless day, proper exposure is achieved with a shutter speed of 1 / ISO, which refers to the sensitivity of the loaded film or the digital sensor of the camera.
• Wait to calibrate your meter on a sunny day. Projected shadows in the field should be austere and clearly defined. To comply with the Sunny 16 Rule Reference, you must perform an accurate calibration under appropriate conditions.
• Dial in your settings on the face of the exposure metric, including the ISO of your selected movie and aperture.
• Maintain the meter so that its light cells face and object of medium tone and reflectance, like a gray concrete wall. Hold the counter right and do not point to the sky or down into the shadows. Alternatively, you can use an 18 percent gray card. Orient the card between the light source and the lens axis and take the reading.
• Read the settings your meter offers at f / 16. If the shutter speed is very close to your ISO setting, your meter is accurate. For example, if you shoot with an ISO 100 movie or have your camera’s sensor set to 100 ISO, a suggested shutter speed of 1/100 or 1/125 indicates a meter is working correctly.
Tips and Warnings
- Interface with your meter between steps 3 and 4 if necessary. A few meters, such as match-need patterns, require simple adjustments to arrive at the correct settings. Consult the owner’s manual for proper operating instructions for your specific meter.
- Find gray cards at all photographic retailers.
- If your meter seems to be inaccurate, bring to a repair shop the unit to see if anything can be done. Most light cell meters are inexpensive and can be easily replaced if necessary.