Thanks for your reply Popo!
In fact I'm talking about actual mirror lenses to be coupled on G1 mfthirds.
I don't have the camera yet, but when I was looking for some primes (and I got some good names between 50 and 170mm) then I realize the 2x crop factor on a sigma 600 (as an example) gives me 1200mm. And is quite cheap.
As many of these come on T mount, and some of them with a 2x T converter, thus gives 2400mm.
I saw many brands between 400 and 600... do you know if are out there some one plus longer, about 1000mm?
In terms of reputation, I think all of sigma, tokina, tamrom, nikon are side by side in performance.
Could you recommend such solution or there are others best by the same price range?
(it's only for beginner purposes and a bit of fun)
During this writing also find this