The EF 500mm f/4L IS II USM is the replacement for the original EF 500mm f/4L IS USM. Its specifications are roughly similar to those of the f/4 Mark I, but have a number of significant differences. The Mark II version contains one fewer lens element than its predecessor (16 as opposed to 17), and is 680 grams (1.5 lb) lighter. It also has a 9-blade aperture instead of the 8 blades of the Mark I.
Read more about this topic: Canon EF 500mm Lens