USING INCIDENT-LIGHT METERS
To use an incident-light meter, hold it at or near the subject and aim the meter's light-sensitive cell back toward the camera. The meter reads the amount of light illuminating the subject, not light reflected from the subject, so the meter ignores the subject and background characteristics. As with a reflected reading, an incident reading provides exposure information for rendering average subjects correctly, making incident readings most accurate when the subject is not extremely bright or dark.
When taking an incident-light reading, be sure you measure the light illuminating the side of the subject you want to photograph, and be careful that your shadow isn't falling on the meter. If the meter isn't actually at the subject, you can get a workable reading by holding the meter in the same kind of light the subject is in. Because the meter is aimed toward the camera and away from the background light, an incident reading is helpful with backlighted subjects. This is also the case when the main subject is small and surrounded by a dominant background that is either much lighter or darker.
The exposure determined by an incident-light meter should be the same as reading a gray card with a reflected-light meter. Fortunately, many scenes have average reflectance with an even mix of light and dark areas, so the exposure indicated is good for many picture-taking situations. However, if the main subject is very light or very dark, and you want to record detail in this area, you must modify the meter's exposure recommendations as follows:
For light subjects, decrease exposure by 1/2 to 1 stop from the meter reading.
For dark subjects, increase exposure by 1/2 to 1 stop from the meter reading.
You will notice that these adjustments are just the opposite from those required for a reflected-light meter. An incident meter does not work well when photographing light sources because it cannot meter light directly. In such situations you will be better off using a reflected-light meter or an exposure table.
If the scene is unevenly illuminated and you want the best overall exposure, make incident-light readings in the brightest and darkest areas that are important to your picture. Aim the meter in the direction of the camera position for each reading. Set the exposure by splitting the difference between the two extremes.