Oh, if ONLY the astronauts had thought to hang some reference-object, such as a 3'X5' piece of nylon (ideally one that's subdivided by high-contrast parallel stripes) in the frame at the same distance as the flag. Then we could have measured the difference in size of the near and far edges and calculated how far it was from the camera.
Of course, if I was going to wish for that, I might as well wish for the astronaut to carry some rectangular object of known dimensions (on his back, say) so that we could calculate his distance from the camera, too.
But that would just be silly.
Oh well...