Phil Stiles , Jan 04, 2000; 02:58 p.m.
Ambient light is non-directional. Incident light meters typically have a white dome, and measure the light falling on a subject. (Generally most accurate, because the reading isn't dependent on the reflectance of the subject.) In-camera meters, and the usual hand held meters, read reflected light from the subject. If it's a cat in a coal bin, they will give you an overexposed negative. If the subject is a polar bear on an ice flow in sunshine, they will give you an underexposed negative. Perhaps your manual is separating "ambient" light from electronic flash, which is another metering mode. And let's not forget there may be several different exposures which are "correct," depending on how you visualize the final result. For a generally accurate incident reading, hold the meter at the subject, and aim it toward the camera. Don't hold the meter in your own shadow. You will be measuring both the ambient light, and any directional light which happens to be falling on your subject.