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Jeff Price December 8th, 2004 12:08 AM

Another long lens question
I've seen varying reports as to how well the AF works with either the 1.4x or 2x tele-extenders and the Canon 300 mm f4 or the Canon 100-400 L. Some say yes, some say no, some yes on the Canon extenders if you tape the contacts. Do any of you have any experience with the AF and the Digital Rebel (or 20D)? Is the AF issue enough reason to go with a Tamron on other vendor's extender?

What about the image stabilization and the tele-extenders? Does it still work?


Rainer Hoffmann December 9th, 2004 05:37 AM

I frequently use the 70-200/2.8 L IS USM and the 300/2.8 L IS USM with the Canon 1.4x and 2x extenders (on the 20D) and it works great. Of course, since the maximum aperture goes down to f4 and f5.6 respectively, the AF is not as fast as at f2.8 or even f1.4.

So, if we talk about the 300mm f4 we end up at f8 when using the 2x extender and this may affect the speed of the AF considerably.

The image stabilization does work with the extenders. I haven't found any issues yet.

Jeff Price December 9th, 2004 09:58 AM

Thanks Rainer but you are using fast lenses. What I've read is that the Canon extenders have been specifically engineered to disable AF on the slower lenses (slower than f4 on the 1.4x). So the 1.4 x will AF on the 300 f4 but not the 100-400. The tape trick is supposed to disable this 'feature' which may have been implemented to save the AF mechanism in the lens from hunting too badly.

What I think I remember someone else in this forum saying is that the third-party extenders don't disable AF - but may not be as sharp.

Rainer Hoffmann December 9th, 2004 10:21 AM

Jeff, what would be the reason to disable the AF when beeing used with the extender? I can't think of one.

Jeff Price December 9th, 2004 12:01 PM

I can't either but that is what some postings suggest Canon has done. They claim that the AF clearly does not work with some of the L lenses, even in bright light, unless tape has been placed over the contacts.

The only thing I can think of is that the AF may not function as well in dimmer light, forcing it to hunt quite a bit putting more wear and tear on the motor.

Then again, I'm also unclear on what the rationale was in making the extender only fit a portion of their lens line.

Rainer Hoffmann December 10th, 2004 03:55 AM

I just looked at the german Canon site. It indeed states, that the extenders are only compatible with lenses f5.6 or faster. The AF will not work with slower lenses. However, no reason for this limitation is given.

Here is a list of compatible lenses (this is from the german Canon website):


Extender sind nur mit ausgewählten EF-Objektiven kompatibel, beispielsweise mit dem EF 70-200 mm f/2.8L USM, EF 70-200 mm f/2.8L IS USM, EF 70-200 mm f/4L, EF 100-400 mm f/4.5-5.6L USM, EF 400 mm f/4 DO IS USM und den Objektiven mit Festbrennweite der L-Serie von 135 mm oder höher.


Your EF 100-400 mm/f/4.5-5.6L USM seems to be one of the compatible lenses. All L-series prime lenses of 135 mm or higher focal lenght are also compatible.

Bernard Diaz December 10th, 2004 01:26 PM

Yeah, I just tried my canon 2x on my 135 f/2 and the AF works fine. However, it does not work on my 400 5.6.
I guess the speed of the lens is the determining factor re: AF.

Jacques Mersereau January 20th, 2005 12:16 PM

I have a 10 D, a 2X extender and the 100-400L. It all works, but as is the
case with all extenders, the image is softer than without. Sometimes
the trade off between closer/softer vs. further/sharper is good (lots of bright sunlight), sometimes not.

Dan Davis March 22nd, 2005 09:09 PM

Canon Web Site
This is copied directly from the Canon US website.

...100-400 L-series super telephoto zoom lens equipped with an Image Stabilizer. The fluorite and Super UD-glass elements largely eliminate secondary spectrum. The floating system also ensures high picture quality at all focal lengths. The Image Stabilizer has two modes and it is compatible with Extenders 1.4x II and 2x II.

If IS doesn't work with the 100-400/extender combination, I doubt Canon would have made the above statement.

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