Tip – Where does the light come from?

If you are using additional light sources (e.g. flash) to illuminate the subject, you should try to ensure that an observer has the feeling that available light is responsible for the light and not simply an electronic flash light. At the same time, the light’s source should be comprehensible for the observer.

Example:
Imagine a researcher in a burial chamber. He points towards an object or a mural. Pass the researcher a lantern or a torch.  Even if this light is insufficient to illuminate the photograph and you would use a flash (with the respective colour compensation disc and possibly slow sync), the visible source of light in the image still suggests it was illuminated by the torch.