Brian,
Do you expect the new 2X Schneider Teleconvertor to work with the 300mm f/4.5 APO? Phase one said it is compatible exclusively with three lenses. That means it won't work with any other lenses. But I don't see why not.
--Steve.
Though I have heard that as well, I have also heard that compatibility is less so focused on whether or not the lens is say, electronically capable of it, but rather whether there is enough room, since the front element of the TC sticks out fairly significantly (possibly even more so then the M645 2x N which I have used with my 300mm f/2.8 and 500mm f/4.5 apo lenses). the M645 2x N will not mount with the 300mm f/4.5 and there is also the issue that you cannot control the aperture. I do not know if the TC will communicate electronically with the 300mm f/4.5 APO but I suspect there should be enough room to possibly mount it, though again this did not work with the 300mm f/4.5 APO and the M645 2x N so it is hard to say at this point, though I will certainly report back.
Of course 2x-ing any of these lenses isn't exactly ideal from an aperture standpoint, but we would hope that as 600 f/8 or 600mm f/5.6 or 1000mm f/8 we would see some improvements with this $2000 USD TC over the inexpensive and older Mamiya M645 2x N which is no slouch as far as a TC can be concerned especially in the demanding arena of MFBD's.
I'm currently working on reviews of the 300mm f/2.8 and 500mm f/4.5 APO in costa rica shooting with the M645 2x N as well as extension tubes, and apart from the images I had somehow labeled, its very difficult to tell whether a TC was used or not since the IQ is very very good (when the picture is in-focus and well exposed of course
)