>> It's a 6 element TC optimized for shorter focal lengths that >> _also_ has a built-in helical extension mechanism.
> That's interesting - I was not aware that it was "optimized for > shorter focal lengths" - I have not read that previously (to your > post) nor have I ever noticed that. My experiences are that the > Vivitar 2X Macro-Focusing TC provides results virtually the same > as the A 2X-S and the T6-2X at infinity (but maybe I'm just not as > discerning as I should be). Oops - a correction: I read your statement incorrectly, Lon. I mistook "shorter focal lengths" for "shorter focusing distances" in my mind. Sorry to muddy up the issue. Well, now that (I think) I have the concept correct, I am still a little surprised by the statement. I have used the Vivitar 2X Macro-Focusing TC mostly on 135-300mm lenses, I guess (and only occasionally on shorter lenses). The only difference I have noticed is that the effect of the helicoid focusing action is more pronounced on shorter lenses, and is less significant on longer lenses. (I seem to recall that 1:1 can be reached only with a lens as short as 50mm, for example.) Is that what you mean by "optimized for shorter focal lengths", Lon? Optically, I have never noticed much of any difference at infinity between this TC and the Pentax A 2X-S. Fred
