I have this combo. I also have the original Kenko Pro 300 DG. The new one that they came out with (DGX) is suppose to have more circuits that allows the camera to communicate better with the lens it is using to transfer the EXIF data a little better than the old. I had ordered this new one a little while back as I was going to stack the two 1.4 TC together to get a little longer reach until I could afford something longer in the Canon lens lineup.

Here's my problem and didn't know if anyone had an answer for this. The new DGX does not work with this lens. It works with every other lens that I own including those shorter than 100mm just like the old. It reports the correct information to the camera so all is good with that. I for some reason cannot get this converter to work with the 70-200mmL f4 IS. When it is connected, the readout shows the shutter speed, but no f value. The camera will not AF with this TC either. Other than taping pins, anyone have a clue how to fix this phenomenon? It works when I stack the two, but doesn't work when it is all by itself with the lens.

Appreciate any feed back. Thanks,