Thanks so much for the reply. Enlightening!
I use Nikon equipment myself, so I’ll try a little experimenting with this stuff.
How do you like the Sigma lens?
CA....
I love it! Honestly, for the money it's probably the best value going for a "long" tele lens. It gets a little soft a full 600mm zoom, so I usually back off a little bit (580 maybe). There are two versions - the "Contemporary" and the more expensive (by $1000) "Sport" version. The IQ of the two is very very similar. From comparison reviews of the two I've seen you'd be hard-pressed to tell the difference. The more expensive one is sturdier (and heavier) with better weather sealing, but I felt that was over kill for me since I'm not a professional. It is available for Nikon mount. I rented one for a week for $65 through www.lensrental.com before I decided to buy it.