There are too many photographs from him for me to have a favourite, but if pushed I could go with this one.
Added note:
By coincidence a debate was going on in the onlinephotographer site at the same time I posted this picture about where Erwitt was standing when he took it. Is it him in the picture, was it staged, was he some kind of voyeur? It turns out the couple were friends of his. However, it does raise issues about boundaries; when does taking a photograph become unacceptable, does how the photograph is used move these boundaries, who decides? Are the boundaries of acceptability wider if its in the public interest, if it is art? Are not most photographs of people voyeuristic in some way?
