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Customer Review

23 of 24 people found the following review helpful
4.0 out of 5 stars Great reach, no auto focus except Live Mode?, December 1, 2011
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This review is from: Canon EF 2.0X III Telephoto Extender for Canon Super Telephoto Lenses (Camera)
I bought this extender with the Canon 100-400 f4.5-5.6L lens, which is listed as a compatible match. It does work, and compared to the $14k for an 800mm lens I can't complain. But Canon should really be a little more clear in their description of what they mean by "compatible". The auto focus does not work when looking through the viewfinder, but it does work in Live Mode, which I find very odd. I have the set mounted on a T2i (550D).

Anyway, so long as you are prepared to manually focus on your subjects, this is a very cost effective way to get the long reach for wildlife shots, etc. Others have already commented on the quality, which for a novice like me, I did not notice any quality degredation from adding the extender

From Canon's support website for those considering this extender: "If an Extender (sold separately) is attached and the maximum aperture of the lens is f/5.6 or smaller (larger f/number), AF will not be possible (except Live mode/u Live mode AF)." So why couldn't they print this in the product description?

And I still don't get why AF will work in Live Mode, but not otherwise--if anyone knows the reason I'd love to hear it :)
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Showing 1-4 of 4 posts in this discussion
Initial post: Dec 6, 2011 9:30:28 AM PST
Alphonsis says:
This is because the AF mode through the viewfinder is Phase Detection. This method basically divides the incoming light into pairs and calculates the difference to know how far off the focus is and in which direction. When the smallest aperture is above 5.6, it means light isn't hitting all of the AF sensors, so it can't calculate AF.

Live View uses Contrast measurement which is a system that measures the difference in intensity between adjacent pixels and looks for the maximum contrast. Usually this means correct image focus. Because this doesn't depend on the AF sensors, it can be used above an aperture of 5.6. It's just slower and less accurate.

In reply to an earlier post on Feb 7, 2012 5:07:44 PM PST
J. Thompson says:
It isn't so much that light isn't hitting all of the autofocus sensors as it is that there just isn't enough light to determine if things are out of focus or in focus.

In reply to an earlier post on Feb 27, 2013 6:51:06 AM PST
Live View focussing can oftentimes be more accurate than phase detection. Even slightly misadjusted optics can cause the phase detection to consistently front- or back-focus, which cannot be corrected on camera bodies that don't support microfocus adjustment. The contrast AF system, while slow, is iteratively adjusting the focus and analyzing the sensor output for maximum contrast. It stops when the image can't be improved by more focussing.

Posted on Mar 27, 2015 5:37:38 PM PDT
I was discouraged about the inablity to AF with this extender until I read this. I just learned that the 1.4 II with my 100 to 400 II WILL AF in live view. Thanks.
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