Canon Extenders don't fit 70-300 f4-5.6L IS ???

Dave Seeley wrote:

I just got my extenders delivered today, and clearly they do NOT fit the lens, as the rear element is far too shallow to accept the extenders. I thought i'd heard otherwise in an on line review, and this shows it with the 2x iii, on The Digital Picture.

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=738&Camera=453&Sample=0&FLI=6&API=0&LensComp=687&CameraComp=453&SampleComp=0&FLIComp=3&APIComp=2

It's the 2x and 1.4x version iii extenders I am trying out. Is there something else for this lens?

You can't just go by the abbreviations on the ISO-12233 page, you have to read the review:

http://www.the-digital-picture.com/Reviews/Canon-EF-70-300mm-f-4-5.6-IS-L-USM-Lens-Review.aspx

"The Canon 70-300 L is officially NOT compatible with Canon extenders. This is the only Canon L zoom lens with a widest focal length of 70mm or longer that is not compatible with Canon extenders (as of review time). I've tried mounting Canon extenders to the 70-300 L - and they do not fit. The rear 70-300 L lens element physically hits the front element of the extender.

"Then, in a major firmware update announcement for the Canon EOS 1D X, Canon included the 70-300 L and Canon EF 1.4x III Extender combo on a compatibility list. Intrigued, I did more experimenting. What I learned is that the 70-300 L's rear element retracts into the lens far enough that, at about 250mm, there is enough clearance for Canon extenders to mount.

"The available with-1.4x focal length range is about 350-420mm and 500-600mm with the 2x installed. Zooming out wider than the 250mm-or-so zoom ring mark results in a physical bump inside the lens. I'm guessing that it is the rubber around the edge of the extender element contacts the rear 70-300 L lens element or its barrel. I do not recommend mounting this combination due to potential damage the to lens. ..."