Q: I purchased a Sigma 500mm f/4.5 APO EX HSM lens, and was told I could use a 1.4X extender with it. Now, I find out the combination will not allow autofocus on my Nikon D200 or D300. Is this an insurmountable problem or is there a workaround?
A: There’s a workaround: manual focus. All Nikon SLRs and DSLRs require maximum apertures of f/5.6 or brighter to autofocus. When you slap the 1.4X extender on your 500mm f/4.5, you’re cutting light transmission to f/6.3—too dim for the AF system, which will automatically shut down. Our advice: Hone your manual-focusing skills. It’s what SLR shooters did for nearly half a century prior to autofocus. (Aren’t most of your subjects at or near infinity anyway?) If you’re addicted to AF, lose the extender and crop into the 500mm image until you find the 700mm angle of view within.
(From Tech Support March 09)_