David H. Bebbington said:
This I think is one of those rare instances where the kind of camera you use has major creative repercussions. I do portraits only occasionally, but when I do I try to use a twin-lens reflex every time. This kind of camera allows you to view the subject AT the moment of exposure, with this I always know when I have got the result I want, with any SLR this is much harder, and with an SLR without an instant-return mirror (such as the Mamiya RB/RZ67, which so many pros use for portraiture) you cannot know what the hell you've got and will feel you need to burn 100 frames of film as insurance!