Testing a photography light meter will ensure accurate exposure readings in the field.
Testing photographic light meters is an essential step to ensure the light meter provides an accurate reading. Light meters allow a photographer to properly expose an image based on the existing light sources and are used as a backup to the light meters present in most cameras. Using a gray card, test shots and an image histogram, you can test the accuracy of your light meter prior to beginning an important photo shoot.
Instructions
1. Use a single lens reflex camera, a SLR, and photograph a "gray card" purchased from a local professional camera shop. A gray card is a true representation of shades of gray and is used to set a light meter using a reflective light source, which is any light that is reflected off the surface of the subject. If you are using a film camera, make note of the exposure settings with a pen and paper for future reference. A digital camera automatically records your camera settings and is best for a quick turnaround and for providing supporting data to test your light meters.
2. Choose a consistent lighting setting, outdoors in the shade or inside next to a window, and bracket the shot at several settings. Bracketing involves adjusting the camera settings, both under exposing and over exposing, to get a wide variety of images. Choose the image that is the most balanced, with a proper showing of light and dark elements. Make sure true whites and true blacks are present in the image. The best way to check the balance of the image is to use the histogram on a digital photograph to see that the peaks of the images are in the center of the scale.
3. Determine the proper settings on the image, and make that your master setting. Place your gray card in the same position, with exactly the same lighting situation, and place the light meter next to the gray card to receive a reflective reading on the light source. Set your light meter to the same ISO and shutter speed setting as your original test image. The light meter should reflect the same aperture setting as your test image.
4. Test the light meter settings if they differ from your predetermined settings. Take an image of the gray card, using the settings from your light meter reading, and study the image results as compared to your original test image. If the image is slightly darker or lighter than the original test image, the light meter is slightly off. Make sure that your light sources are consistent and controlled, as this will change the settings determined by the light meter. Using a digital camera for this test ensures that all the settings are consistent and allows for a rapid turnaround on the test. Using film requires days of testing, due to film processing times and recreating the lighting situation each time you conduct the test. Bear in mind that printing machines also vary, so unless you are an expert at reading exposed negatives, the histogram from a digital camera is your most reliable option for testing.
Tags: light meter, gray card, your light, test image, digital camera