It is a 1,200mm lens in 35mm terms and the bluebird photo was taken at 1/350 sec and I have no idea what the f/stop was since there is no adjustable aperture in a mirror lens.
OK so about twice the attached (effective 630mm... Sig 70-300 full plus Tam 1.x4)
(Plus hand held with 10D SR only compensating for 450mm, due to the 1.4x)
OOPS I for got though all that SR stuff is a bunch of marketing whooey. :?
Actually just because it was real cheap, got a K mount to T-2 adapter.... might just have to see what my little 70MM Mead might look like as a lens.