Well, I’d say subjects that the average person would find very difficult to catch….be it a rare occurence for example.
So, say, Monet is out, ‘cos he just painted stuff about folk lying around by rivers and other subjects that are not exactly earth shattering?
Now you’re being silly, I haven’t seen any photos taken by Monet.
Given that a photograph of a person is moment in time and given that no two moments in time are ever the same, any photograph of a person is thus a rare occurence.
True, however I would expect that the subjects in this particular situation were sitting still and despite the fact they were getting older, there was probably at least a few hours before anyone would have noticed!