How can I really answer this question? Why natural light only? Portraits are portraits, regardless of whether the light is the sun or an electric light, and light is light. "Natural light portraits" is such a meaningless technical category. Why make that technical distinction between one source of lighting and another, as opposed to a conceptual one? Talk about meaningless...this is like asking who the masters of telephoto portraiture are, or who the masters of overcast portraiture are. I will tell you that anyone whose portraiture is best described in one short phrase by a technical term is nobody who is worth talking about. Now, "natural portraits" I could understand...