300/4 IS w/2X

Recently bought a 300/4 IS used and like it alot. Just tried using it with my Canon 2X (1)which cancelled the AF. Ran a search but nothing popped up. If this is the case, then does it work with the non IS 300/4?

depends on what Camera you have..I anything but the 1series wont do AF with anything smaller then f5.6
I think putting a 2x on there makes it like an f8 or something.
so you cant AF..if you used the 1.4x it would still work cause it would be f5.6
I looked into this cause I was thinking about getting a TC for my 70-200f4l and the 1.4x is the only one I can use and still use the AF

On a 20D. The f/5.6 makes sense since it works with my 70-200/2.8

Yes, anything slower than f/5.6 won't AF on a non- 1 series. You lose 1 stop of light on the 1.4, and 2 stops on the 2x.

Yes the 1.4X will work on any body but the 2X will only AF on a 1 series body. The lens gives fair IQ with the 2X so it is worth seeing how well the MF works for you.

Check out David Chauvin's galleries at http://www.pbase.com/d​gc_arts/south_la_birds
He uses a 300 f4 with 1.4x and 2x very successfully.

In order to work with AF and a non-1 series camera with an f/8, you must tape the 3 pins to avoid send the real f/stop info to the camera and that avoids AF.

To be more clear, I have a Kenko x2 TC. The side atached to the body has 8 pins, the side atached to the lens, 11 pins. You must tape the left most 3 pins of this side (LENS SIDE, total 11 pins) in order to be able to AF. The AF speed is slower and it will hunt in low light, but it works.

I have found in my notes the possible reason under this practice, so I copy it literaly:

"Well, why are you taping the pins? Is it so the lens doesn't know that the extender is in place, and thus will report the f/number of the lens, and not the realf/number of the combination, and thus make the camera think that the aperture is larger than it really is, so it won't suppress the operation of the AF?"

I am not pretty sure if this stament is correct at all, but it serves to explain it.