#1. "RE: Ideas and advice on the D7000 and the 500mm" In response to Reply # 0
Depends on your subject, the conditions, and the conditions of display, meaning the eventual use of the image. If your subject is a little bird, like a sparrow, you're probably way too far at 30 feet. If your subject is a moose, you can't even frame him/her with the environment at 50 feet in a 500mm - probably 50 yards is about as close as you'd ever care to be with that lens.
The conditions matter too. If there's mist in the air, or let's say there's a ribbon of blacktop between you and the subject and it's 100F and cloudless - 20 feet might be too far, since the intervening air is so disturbed or so full of stuff that you may not be able to resolve almost anything. Under ideal conditions, I've been able to resolve eyelashes on a racing car driver through the face shield at 100-120 yards, although that's with the 400/f2.8 and TC-20eIII (on FX, with only a 12mp sensor).
The eventual use of the image matters a lot: if the image is going to produce a gallery quality 30x40" print, you have a lot less margin for error than if you're going to email the image or use it on a web site.
It's worth mentioning, though, that this is really not the limiting factor most of the time. Usually I have to look hard for a really good shot since nearly always the limiting factor in my photography is my technique. I frankly do not worry much about image quality other than paying close attention to technique. I'm much more interested in getting the subject framed properly, in good focus (I do not think it's that easy) and well composed. If I've done those things, even reasonably decent equipment produces a good result. Top-drawer stuff like a 500/f4 or 400/f2.8 is just a bonus.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!