Title says it all really. Huge formats obviously mean long focal lengths - in some cases longer than you can find proper compound lenses in. Large achromats on the other hand are available in the 1800-3000mm range and not too expensive in modest diameters. What sort of usable coverage can you expect from them? Is stopping them down necessary to have any chance of a reasonably sharp picture (just talking basic contact printing standards here)? How about using them with smaller formats that just use the central few degrees of the lens' coverage - similar to the way they were designed to be used with telescopes? Would love to hear the thoughts of anyone who's tried this, before I go sending a few hundred dollars off to China for something that could turn out to be completely useless.