Quote Originally Posted by Diapositivo View Post
I find what 2F/2F says very convincing. Just to visualize it, imagine a portrait where the main light is a very lateral one (going by heart, Churchill's portrait by Karsh). Pointing the dome at the camera would bring overexposure. One does not want to "average" light and shadow, but to expose for light and then devise and use shadows to give more or less volume, "drama" etc. to the subject, so it is the main light that should drive the exposure.
Yes, let's visualize it.

Rule number one with light meters is to always remember that regardless of what is measured the meter always displays a reading that should place that measured brightness as middle grey.

By pointing the incident meter at the source light you are choosing to measure something closer to highlight placement, as you might with a spot meter.

Pointing an incident meter at the source rather than the camera will provide a reading that creates an underexposure.

The only real exception to this rule is for sources like ring lights that are directly in line with the camera lens axis.