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Old Jun 28, 2005, 4:06 AM   #1
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Hey all Digi Photographers,
I have a Canon 20D with 70-200 F4L and loving every momemt with it. I am a aviation enthu and would like to extend my possibilites with my current lens. I am planning to buy the 2X teleconverter either Canon or Sigma. My only question is will the autofocus work when stacked with the 2X along with my 70-200F4L lens? Also one another question would be; which brand to choose? I am not pretty picky on the price, but saving $100 on the sigma would help me invest on a very nice bag!

Please do advice.

Thanks in advance.
Regards,
Vasanth
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Old Jun 28, 2005, 10:21 AM   #2
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The 2x TC will not auto focus with the 70-200L f/4. Even in bright sunlight, I couldn't get it to do so. Canon recently had literature out there that said it would and would auto focus, but it won't (no surprise). That was tried using the Canon TC. The Sigma EX TC should be the same. As which one to choose...you can't go wrong with wither one. The 1.4x TC at f/5.6 will give you a little more reach and will still AF. The 2x could still be an option with that lens even though it would be f/8 and sharpness wouldn't be the same. It would be a case where you would master manually focusing which isn't really all that bad. Either way, good luck!
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Old Jun 28, 2005, 11:11 AM   #3
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I think it is a case of the firmware seeing something larger than f5.6 and giving up without even trying :-).

There is a trick you can try, if you cover the right pin on the TC with a bit of tape the information about the TC is not realyed to the camera and it will try to AF, rather badly though :-).

Not sure which pin, there was a thread about it a while ago.

Or use a generic TC like Kenko or Tamron which don't bother to tell the camera they are there. Image quality is much worse with the generics though.

Peter.
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Old Jun 28, 2005, 11:20 AM   #4
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I have heard/read the same thing about covering the pin(s) to achieve AF. As far as the Kenko/Tamron TC, the white versions of both are meant to act just as a Canon or Sigma EX, but the cheaper ones will AF as long as they have enough light. I used a 2x Tamron on a Tamron 70-300 f/4-5.6 and it auto focused wide open and zoomed in. Naturally the performance and results were only so-so, but it IS an option.
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Old Jun 28, 2005, 12:02 PM   #5
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I recently bought the cheap Tamron 1.4X TC for use with my 20D and EF 70-200 f/4 L. It works great. It doesn't connect electrically to the camera, so it sends no information concerning apature (or anything else), negating any requirement to tape pins. Downside is the EXIF data isn't correct.

Can't say whether the Tamron 2.0X would auto-focus or not since it doesn't report back the apature to the camera.

This is an inexpensive ($80) TC that gives a nice boost in reach to the lens. If you want a tiny improvement in performance at four times the cost, get the Canon. Otherwise, this is a very satisfactory solution.

Be sure to use a tripod, because it doesn't take a lot of camera shake to screw up a frame at extreme zoom.
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Old Jun 28, 2005, 1:52 PM   #6
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As far as I know, the pin taping trick only works with the 1.4TC and the 100-400. At least, that is what it said on the web page where I learned the trick.

Unfortunately, it really didn't work well enough to be useful for anything that moved. It hunted so badly as to take several seconds to achieve focus, and it did a lot of hunting back and forth to achieve focus. For me, this made it basically useless.

To confirm what others said, you can't answer the question:
Does the 70-200 f4 AF with the 2x TC?

It can't be answered because you need to know the camera body. With the 1-series cameras, it would AF, with the non-pro models it won't. So with your 20D, it won't AF because the firmward would see an aperture greater than f5.6

Some after-market TCs do work, as pointed out, because they don't adjust the aperture value... the camera doesn't know the aperture is greater than f5.6, so it still tries to AF. I doubt it will do it very fast, but it will do it.

What Wildman did should work fine. The 1.4TC would make it an f5.6 lens, and that will work well. I don't know if that TC is multi-coated, so you might have some issues in certain situations. For example, if it reduces contrast then that would slow AF. But it should work. It will, of cource, reduce quality... giving you a softer image.

The real trick would be to get the Tamron 2x TC and make that work.

Eric
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Old Jun 28, 2005, 7:01 PM   #7
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This thread might help...


http://www.stevesforums.com/forums/v...mp;forum_id=65


mactek
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Old Jun 30, 2005, 3:50 AM   #8
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hi all,
tnx for your valuable replies ... yet i have one more clarification - http://www.bhphotovideo.com/FrameWor...xExtender.html ..here the compatibility says possible with a * onto it. so does it mean the AF works flawlessly only with the EOS-1v and EOS-3?

Thanks.
Regards,
Vasanth
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Old Jun 30, 2005, 7:01 AM   #9
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vasanthd wrote:
Quote:
..here the compatibility says possible with a * onto it. so does it mean the AF works flawlessly only with the EOS-1v and EOS-3?
Some cameras AF can focus all the way to f/8, but the 20D can't it'll stop at f/5.6:
An f/4 lens becomes f/8 with a 2x teleconverter, the camera knows this when it communicates with the lenses -> so for cameras capable of focusing to f/8 it will work, and for cameras with a limit of f/5.6 (ie the 20D) it will not - unless you stop this info from flowing between the lens to the camera by taping the pins (or through a TC without them), but then the focusing becomes unreliable...
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