can I use a 2.0 magnifier with a 70-300 zoom

[Kip]

Yes, but depending on the camera model and aperature of lens; you may only have manual focus available. My 7D will still auto-focus on center with f/4 or faster lens.

[cicopo]

Good in depth info here.

http://photography-on-the.net/forum/showthread.php?t=41922