Is it worth the extra 200mm if you already own a 100-400? Will it yield any better result than cropping the 100-400 @ 400mm? I am hopeful that there are either some users of both, or users of the Sigma 150-600 that can speak to its capability at 600mm. In comparison, I am very pleased with the performance of the 1000-400.
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http://www.greinerstudio.com
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Olympus O-MD E-M1 ii
Panasonic Leica 12-60 f/2.8-4
Olympus 60 f/2.8 macro
Olympus Pro 40-150 f/2.8
Pansonic Leica 100-400 f/4-6.3
Olympus 300mm f/2.8
I used the sigma 150-600 C via a metabones smart T on my EM-1 and it was a superb match and value for money.
If it hadn't been for the fact the weight was too much for me to hand-hold (I'm old and decrepit) I would probably have kept it to run alongside my 40-150 pro
As it is, I went for the 100-400 which is the best compromise between price, weight and performance for my use
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So much to learn, so little time left to do it!