Gary, to oversimplify, a cone of light comes out of any lens and travels to the film plane. If we know what angle that cone is it's relatively easy math to figure out what the lens will cover. So if you have a lens that gives 105 degrees you can draw on a piece of graph paper a 105 degree angle coming form a source (the lens) and see approximately how far away the lens would have to be from the film plane to cover a 600mm diagonal. A 60 degree lens like a tessar has to be quite a bit further away before that angle can cover 600mm diagonal. That's why it takes a 450mm Nikkor to get the same coverage as a 210mm Super Angulon. Symmar's are 70 to 72 degree lenses. So a 480mm Symmar would cover a 16X20 piece of film. G-Claron's are roughly an 82 degree lens at small f stops. So a 355 G-Claron will cover a 600mm diagonal at infinity. Hope that helps.