Incident Light Measurement

Chuck isn’t this kind of basic? Yup, you are right, it is basic, and because it is basic sometimes we forget the power and simplicity of it. Not only that, many photographers are so locked into using their camera’s light meter, particular on automatic, spot mode.

Is it always the best?

Or is it just easy?

Are you trying to create great images?

Or just capturing the shot?

There are two major methods of measuring light in photography: measuring the light reflected from the subject, or measuring the light falling upon the subject. No matter which method of measuring exposure is used, it must be remembered that exposure really is determined by the amount of light reflected to the camera by the subject.

The important question to ask is what is your goal for the image?

What area of the image do you want to stand out?

Whatever amount of light that is reflected into the meter is read as 18% grey, so if you photograph something that is dark and you measure that area, it is automatically measured at 18% grey. In the finished print it will look too light.

Likewise, if you are photographing something light, this also is rendered at 18% grey. Opposite to the first image, we end up with an image that is too dark.

True, you can change the effect in printing or your digital imaging software, but the result will not be a faithful rendering and other values in the image will be incorrect.

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 back lighted 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.

*You can do this with your camera too, just face the source of the light, shield the lens from the direct light and take the reading.*

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.