Like anyone else, every now and again I come across old photos of family holidays. There are quite a few from the mid-1970s, mostly taken on Bournemouth beach. Obviously, I look at the faces of my parents, now gone, or of me and my sisters – but sometimes I find myself focusing on the other holidaymakers in the background, in their own deckchairs or building their own sandcastles: strangers who, by chance, were caught by our camera and preserved for ever. There is something of that sensation – the random serendipity of the tourist’s lens – at work in a...