If any one is in Chicago there is a Callahan show at the Art institue. I think its for a couple more weeks? I talked my wife into going with me a few weeks ago(she doesn't normally go to something like that) and she noticed and commented on how varied his subject matter was. We both liked the short explanations that callahan gave for some of his prints. He wrote that he was very unsatisfied with his early versions of the trees print and realized to satisfy his vision he was going to have to break with the current tradition at that time of a full range of tones and detail in the shadows and print it much more graphic. It sure worked in this case.