>Would it not be worth considering getting a 1.4x >teleconverter for the 300mm lens and shooting ff with it?
I guess I was trying to make a super telephoto lens out of a 300mm f4. I have an AFS TC-17EII that will allow the D600 body to autofocus in reasonable light levels. In a few experiments I tried it worked fine. I was thinking I could (seemingly) appear to get more reach by using DX cropping but as I have learned from the foregoing there's no free lunch. I'll be satisfied with 510mm at F6.8. Again thanks to everyone who chimed in. I learned a lot. (and the release of the D7100 makes for another story ). cheers
Cold weather photography makes a VR lens your best friend. Think violent shivering. :-)