Thanks for the reminder, LD.
Coincidentally, there's also a J M Cameron show at the Media Space in the Science Museum just over the road, also free I believe.
And once you've had your fill of Cameron's portraits, pop along the road to the Royal Geographical Society and you'll catch the exhibition of Frank Hurley's images from Shackleton's Endurance expedition (I think these will be new prints from scans of Hurley's originals, but still worth a look).
I intend to get to at least two of the above before they finish, and hopefully find the time for a little camera walk around Hyde Park too.