Yousuf Karsh (1908-2002) a Canadian Photographer of Armenian descent, is considered to be one of the most famous and accomplished portrait photographers of all time.
The title of this article was a quote that appeared in the 'Sunday Times' and gives you a clue of the significance that he has had on portrait photography and the respect that he commanded amongst the greatest icons of the world. During a career that spanned more than 60 years he had more than 15,000 portrait sittings producing more than 150,000 negatives, which is absolutely staggering !
He used almost exclusively an 8x10 large format camera and has photographed the likes of Winston Churchill, Fidel Castro, Albert Einstein, Ernest Hemingway and George Bernard Shaw just to name a few.
Currently in Chicago, there is an exhibition of more than 300 of his prints at the Art Institute of Chicago, just in case you happen to be around that neck of the woods.
The OnlinePhotographer has a very interesting article thats well worth the read, and, an absolute must, is a visit to the Yousuf Karsh website itself where you can read a short comment of a number of his most famous portraits explaining what happened during the portrait sitting (absolutely fascinating), there is also a quite extensive wikipedia entry about him too.