How to Check Light Meter Calibration

Is the light meter in your camera working properly?

Relying on blind faith that your camera's light meter is calibrated properly can lead to incorrect exposures that could happen on a regular basis, resulting in poor-quality pictures. There's an easy way--known as the sunny f16 principle--to gauge the performance of your camera's light meter using a grey card or similarly toned material.

Things You'll Need

• 8-by-10 gray card

Instructions

The "sunnyf16 rule" test

1. Find a spot in full, direct sunlight (sometime between two hours after sunrise and two hours before sunset) where you can place the grey card so that it offers an unobstructed field of vision to your camera. When you look through the viewfinder all you see is the grey card.

2. Set your exposure mode (for those cameras that have automatic exposure) to aperture priority, with the aperture chosen as f16. For manual exposure cameras, just set the aperture to f16. Position your camera so that the grey card fills its field of vision.

3. Obtain your camera's estimation of the proper exposure. For auto exposure, just depress the shutter release button partway and note which shutter speed the camera wants to use. If you have a manual camera, watch the exposure indicator as you change the shutter speed, and note the shutter speed that corresponds to what is indicated as a correct exposure.

4. Determine whether the exposure is correct. According to the sunny f16 rule, the correct exposure under the conditions above will be at the shutter speed that corresponds to the reciprocal of the ISO speed of the film being used (or in this case the ASA/ISO setting of the camera). Note: Due to the vagaries of the different settings found on different cameras, the shutter speed might not exactly match the reciprocal of the ASA/ISO setting of the camera, but it will be close enough to indicate the meter's accuracy.

   For example, if the camera is set for ASA/ISO 125 and the test resulted in a shutter speed of 1/125 then the light meter is behaving accurately. Similarly, if the shutter speed had been recommended at 1/60 of a second for the same test, it would mean that the light meter in your camera was returning results that are off by one f-stop, and you'll need to make adjustments.

5. Use one of the following three methods to obtain the correction. Set the ASA/ISO on the camera one stop lower; set the aperture one stop lower in aperture-priority mode; or select a shutter speed that is one stop lower in shutter speed priority mode.

   (Note that changing the f-stop lower means reducing the exposure, so the ASA/ISO setting would be set for a faster film, or the aperture would be smaller, or the shutter speed would be faster.) Of course, your meter might be in error in the other direction, which would require correction in the opposite direction, i.e. stopping up rather than down.

Tips & Warnings

• Choose a day for the test when the sunlight will be optimal--a dry day with no smog or other atmospheric pollutants.

References

• Exposure Calibration, Bob McBride: The Nature Workshops