I have the 250mm Imagon. As I recall, the lens is f4.5 without any disks in it. The lens was made in several lengths. The 200mm was used on medium format, the 250mm on 4x5, the 300mm covers 5x7, and the 360mm covers 8x10. This seems to be a challenging lens to learn, but then again I don't shoot portraits and have been using it for b&w landscapes.
Kent in SD