Infinity focus & extension tubes

[Dr Croubie]

I've seen some debate as to whether or not
a 12mm extension tube could be used on a very long telephoto lens and
still retain infinity focus, but I've never seen proof of it working.

and
just to spam this thread with another reply: i've got a 250mm f/3.5
soviet MF prime, and two 2x teleconverters (making 1000mm f/14). if i
could find a way to mount it on a tripod i'd give it a go when the sun
comes out...