All meters are programmed to assume the subjects in a scene are middle gray, which is to say that they reflect about 18% of the light that strikes them. If a subject is very light, like snow, or very dark, like a man wearing a black tuxedo standing close to the camera, the meter will be “fooled” into providing an inaccurate exposure. By calculating the exposure that will make the snow appear middle gray, it underexposes the subject. If the man wearing the tux fills much of the frame, a typical in-camera meter will overexpose the picture because it assumes the black fabric is gray.
One reason why built-in meters fail so often in giving you an accurate reading is that they don’t allow you to read a very narrow portion of the composition. Even in spot mode, they read 3% to 5% of the scene, and that is not specific enough to take an accurate reading in many circumstances. A handheld meter reads the light in one degree, i.e., .05% of the composition. This means that you can choose the portion of the picture that you have identified as middle gray and obtain an accurate reading from it.
When you use a handheld meter in reflected mode, you simply identify a middle-toned area of the scene and point the meter at that area. If your meter has a viewfinder, you can look through it and position the viewfinder’s target circle over the middle-toned area before you take the reading. When you push the button to take the reading, the f-stop/shutter speed combination that you need to use to get the proper exposure will appear instantly on the meter’s LCD. The accuracy of fine meters like Sekonic’s falls within a tenth of an f-stop. A typical reading, then, would look like f/8 plus .7, which equates to f/8 and about two-thirds. If you want to use a different lens aperture or shutter speed than the ones displayed—for example, for more depth of field or to creatively blur a moving subject—then you can simply rotate the main dial or press the arrow buttons on the meter to see the f-stop/shutter speed combinations that will achieve the correct exposure.
When you use a middle-toned portion of a scene as the basis for the meter reading, all of the other tones fall into place correctly. For example, in the shot of an iceberg below, I used the dark gray clouds as the middle tone, and in the picture of the trucks, I had a choice between the dark red paint or the rich blue sky. Both of those areas are middle tone, and reading them allows the entire picture to be properly exposed.
The incident metering mode is not available on cameras. It is a function that is available only with a handheld light meter. In my opinion, this makes a handheld meter like any of the Sekonic models worth its weight in gold.
In incident mode, you use the lens covered by a white hemispherical dome (below) to read the light falling on your subject, instead of measuring the light reflected from it. The difference is that in reflected mode, the meter can be fooled because a white subject and a black subject reflect light very differently. When using incident mode, how light or dark the subject is doesn’t matter because the meter is reading the light emanating from the sky or from an artificial light source before it gets to the subject.
To use the incident mode, you select it on the meter and then hold the device in the light that is falling on the subject when you take a reading. If you and the subject are both in open sunlight, you can take a reading from where you’re standing, near the camera. However, if you are standing under the shade of a tree and your subject is being illuminated by direct sunlight, you must walk over to the subject–or at least into the sunlight–so that the meter detects the same level of light that is falling on the subject.
When you are taking an incident reading from the subject’s position, the meter should be held so that the white dome faces the camera lens, not the light source. This ensures that the light strikes the dome exactly as it is falling upon the subject. For example, if you are photographing a face that is side lit, as with the young Indian girl in the photo below, the light must hit the meter from the side. Hence, the dome must face the lens just like the subject’s face does. By positioning the meter correctly, you will allow it to calculate the exposure correctly, regardless of the colors or the reflectivity of the subject.
Read more at http://www.sekonic.com/whatisyourspecialty/photographer/articles/how-to-use-a-handheld-meter.aspx
When reading a hand-held light meter, make sure the ISO setting is correct and point the cone of the meter toward the subject to get an accurate reading. Learn how to read a hand-held light meter with the tips in this free video on photography tips from a professional photographer.