Whenever I get metal parts stuck together, I freeze them awhile in hopes the metals will shrink and enlarge any holes involved. This was recommended to me but has not yet worked in practice. I do it becaue it's cheap and easy. I've been told that when metal rings shrink, the diameter of the opening increases because the metal pulls inward on itself rather than toward the center of the circle. In a high school science class, this was demonstrated with a ring and a ball and a bunsen burner. Apparently, the perimeter of the ring doesn't decrease much, so that as the metal cools, the opening can expand while the ball shrinks. I've never tried cooling a lens, but it shouldn't hurt it.