I currently have a 100-400L and a Canon 2x extender and was wondering if it was only canon extenders that you lost auto focus on, or all lenses slower than 2.8 regardless of the extender used.
The 100~400 is f/4.5-5.6. With a 2x teleconverter you have, effectively a 200~800 f/8-11. The EOS 3 AF functions at an effective max aperture of f/8, but only the center AF point does. As far as I can recall all other AF bodies lose reliable AF if the effective max aperture is below f/5.6.
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regards, Henry Posner B&H Photo-Video, and Pro-Audio Inc. http://www.bhphotovideo.com
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