40D camera, EF 70-300 IS USM lens, Kenko 1.4 300PRO tc not working.

[dentheman]

Hello, I have just purchased a Kenko 1.4 300PRO teleconverter to use with my 40D and 70-300 f/4-5.6 IS USM lens.  At first it worked OK, but autofocus was iffey.  Now sometimes when I press the shutter halfway, I can see the mirror flip part way up and back down, and I sometimes get the error that the lens and camera are not communicating-clean the contacts.  I have cleaned the contacts, but that hasn't helped.  If I use the lens without the TC it works fine.  Supposedly this lens and TC are compatible, but I'm not so sure now.  Do I have a bad TC, or are they just not compatible?

[Bob Atkins]

AF will indeed be "iffy" since the lens is too slow at f5.6 to properly support a TC.

If it ever worked then it should stilll work, or at least it shouldn't report an error. AF may well fail, but that's to be expected. AF may depend on the contrast of the target and the light level, so it may work in some situations and not others. It should always work in manual focus mode though.

You might want to contact Kenko and see if they have any reports of incompatibility, but I haven't heard of any particular problems.

[dentheman]

Thanks, Bob.  After further investigation, what I thought was the mirror partially flipping is actually the image jumping around in the viewfinder, so power to the image stabilizer is being intermittantly interupted.  When the IS is off the image quites jumping. I obviously got a bad teleconverter, and will be sending it back.  As far as the autofocusing, I was surprised that it autofocused at all at 300, but it does focus quite well at 70.  Not a big deal since I got my first SLR back when everything was manual focus.

[dentheman]

THKphoto has not replied to my two emails.  Isn't there anyone out there with the same camera/lens/teleconverter combination that can help me with this question?