Hoopers Gallery at 15 Clerkenwell Close is worth a visit. The current exhibition is by 4 photographers from Romania, California, Ireland and England. It's a mix of colour and black and white. There is a show of Edward Westons work I think in the Barbican and the Photographers Gallery in Great Newport Street is always worth a look. The best place of all IMO is the Victoria and Albert Museum, they have a print room on the 5th floor and probably the best collection of prints by the masters that can be found anywhere. It's worth the visit, I go every time I have a few hours to spare when I'm in London.