Originally Posted by
William W
Hiya,
There are various points of incorrect data contained in Canon's Data Tech. Sheets and User Manuals: no doubt oft these inaccuracies are to protect their product. Examples of inaccuracies are especially prevalent in Canon's data / compatibility lists for their Extenders EF.
I think you'll find that both your Extenders EF MkIII will mount and will 'work' with your 70 to 300L lens, BUT there is a (big) caveat: you must set the lens at 300mm to mount the Extender and you must use the lens at 300mm (actually the rear of the guts of the lens hits both the Extenders when the lens is zoomed wider and that impact occurs at around FL = 260mm).
The above applies to both Extenders EF MkII and those are what I've used: the physicality at the Lens' Mount end of the MkII and MkIII extenders is the same, that's why I think that your MkIII should mount and should work as described.
I have not used the Extenders EF (original) - (not many of those around now I think), but again the physicality at the Lens' Mount end is the same (if my memory is still OK)
So whilst Canon data prescribes incompatibility and non-mounting, literally that is not correct: the fact that it can be done might be useful information.
WW
Addendum:
Yes, concur good value for money lens. Yes price drop inevitable, if not already happened.
Also concur with Richard's assessment to avoid the Canon 75-300mm f/4-5.6 lens.