Originally posted by JayR ok, now im really confused
and what specifically is a 'rear converter' TC? if anyone can help!
(you can see i never bothered with TCs in the past!)
Rear converter = Tele converter. It is a set of optics that attach to the rear (camera end) of the lens, as opposed to those attachments that go on the front of the lens. The front converters are less than satisfactory when used on DSLR bodies.
Some rear converters will not physically fit to a given lens, which is what Lowell was pointing out. For example, the Pentax 1.4X-S and 2X-S converters are intended for lenses shorter than about 300 mm focal length. The Pentax 1.4X-L and 2X-L are for use with lenses over 300mm. The catch is, that both the -L converters protrude past the lens mount. I cannot use them with my M 400 because the throat of the lens is not wide enough to allow the protrusion to enter the lens, and must make do with the 1.4X-S, and that is not optically ideal.
There are other lenses that have a throat that is large enough, but the length of the protrusion will cause the converter to bump into the rear lens element, which is close to the lens mount.
I hope this clears up some of the fog for you.