CanonFanBoy said:
I regret spending the money on my 2x III. I've tried it on my 400mm f/5.6L (manual focus only with the converter) and my 70-200mm f/2.8L II and it just doesn't impress.
I need to try it on my 135mm f/2L and see how that works. I think that would make it an f/4? That is the only lens I have that I think it would be useful for. That makes the lens an apparent 432mm f/4 (on crop) and a lightweight combination faster than the 400mm f/5.6L
However, I have not AFMA any of the above combos... and I should get around to that. I won't for the 400 f/5.6 though.
The 2x TC's are really best suited to only a few lenses. A 2x TC will give good results with the 'great whites' (200/2 through 600/4, ok with the 800/5.6), and the MkIII TCs should be paired with the MkII lenses. The 70-200/2.8L IS II does well with the 2xII/III, results are equivalent to the original 100-400L. The 100-400 II is better, and either 100-400 handles better than the 70-200 II + 2x, so the only reason for the latter combo is if you already own the 70-200 II and only occasionally need 400mm.
You likely will not be happy with the 135L + 2x.
For combos that should give good IQ with a 2x, it's definitely important to AFMA the combo if you're going to use phase AF, the TC can significantly change the optimal adjustment value.