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.
Sigma lenses have proven to be excellent choices for me since my first (of now 4) lense. Pleasing photograph results has netted my confidence and trust in the name. Certainly a bargain investment in my unprofessional opinion.
After hearing all the praise for Sigma's Art line of lenses I decided to purchase the 35mm and I could not be more happy with my purchase. It is sharp wide open and the bokeh is beautiful. I'll be adding other prime lenses from their art line to my camera bag.
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