Of course, the 50-200 is fairly long is at 200mm, is F/3.5. Then either a 1.4x or 2.0x teleconverters. The 1.4 is said to work very well providing a 280mm length at F/5.0 I think ... the 2.0x teleconverter would make the 200mm a 400mm, at F/7.1. Most reports say F/7.1 is a bit marginal, but in bright light, perhaps not. Of course, 800mm FF is very good reach indeed. I have a 50-200mm, and would love to try it ... but such a 2.0 converter is not so cheap! The 280mm at F/5.0 is said to be terrific. I guess too, it shows what a good job the Panasonic has achieved, with a much lighter lens, at F/5.6 while at 300mm, yet its half the weight of the 50-200mm, and it has OIS. However the 50-200mm is telecentric, and a very fine lens indeed.