A current ebay ad lists a 300mm Nikkor-W as yielding 420mm image circle at f22. I really don't understand a whole lot of issues related to this. Can any of you folks out there help me a bit here?
Many thanks in advance!!!

1. Imagine I project an image generated by the lens onto a huge sheet of groundglass. I'm assuming f22, infinity, acceptably sharp focus, and critical viewing of image with a loop.

2. So I get my 420mm of useable/good quality image circle which the manufacturer is promising in his, I assume, conservative specs.

3. My generic question about lens performance: as I stop down from 22 toward 64 or from 22 toward wide open, what happens optically? What happens to my useable (above) area? Will it tend to lessen in size (bigger/ smaller?), degrade in quality, or what?

4. Put another way, folks/manufacturers will say of a particular lens, "covers xxx at so-and-so f-stop at infinity." Does that mean that at other than the quoted/published f-stop (like the common f22 above), the total projected image circle is literally smaller or larger? Or does the image circle physically stay the same size but with major quality changes within the, say, "420mm image circle."

Just trying to learn. What primer books might I check out? Many thanks!!!

Robert McClure - Atlanta