I am using this 2X extender with my 135L, and I must say that after a little micro adjustment of +5, this is incredible! I now have a 270mm f/4 lens, and the images are very, very nice. Wide open (f/4), there is a little softness or dreamy effect, but stopped down to f/5.6, images are extremely sharp and contrasty. Yes, this will enhance any flaw in your lens, but with higher quality lenses, should not be much of an issue. AF is fast, I have not had any problems with its speed or accuracy. Overall, I am extremely pleased with this extender.
My wife uses this lens for portrait and flower pictures and they look amazing. We tested several lenses in the same price range before buying this one and while they were all great this one was the best for our Canon 7D and light conditions.
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