Hello Jess, and welcome to the friendly Camera Labs forum!
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Popo has already pointed you in the right direction. I personally tested the Sigma 120-400 OS which is a close cousin of the 150-500 if you look at the design.
I gave the 120-400mm back after intensive testing (see my review here
) because the OS was only 1 stop effective and there where some halo-like effects around bright subjects. I assume
the 150-500 is similar in this respect.
Well, if you shoot mostly with a sturdy tripod, the efficiency of the OS is of no consequence for you. But if you're handholding much (as I do) a good working IS is tantamount to getting sharp images.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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