I don't think photos at museums are flat, but do find they require a lot of concentration to read – especially vintage prints in small frames. After a big photo show at SF Museum of Art, like the recent one of Gary Winogrand, I want to go to some abstract expressionist paintings to rest my eyes.
Wouldn't mind seeing photos mixed with paintings, Westons and Stieglitzes with Marsden Hartley and Charles Scheeler, Robert Mapplethorpe with Helen Frankenthaler or Joan Mitchell, Eggleston with Robert Ryman etc.
Joel Sternfeld gave a talk here a few weeks ago and said that with the quality of book reproduction approaching that of gallery prints, that he's beginning to prefer books – and that books are the way his photos work best.
And maybe a photo book does begin to capture the kind of time in a painting.