I have been a long time owner of the Canon EF 100-400mm lens. I have found the lens to perform quite well, especially for a f/5.6 lens. It is a popular “L” series lens because of its long reach for a reasonable price. Last year I upgraded from the 40D to the 5D mark III, and had been missing the extra reach I was getting by putting a 400mm lens on a APS-C sensor. It was effectively giving me 640mm when you account for the crop factor.

Now that I have have the 5D mark III, I have limited options to gain that reach. Basically the only option is to buy a big prime lense, and who can afford those? 600mm or 800mm for $13,000? no thanks.

So this situation has got me looking into Teleconvertors. Few hundred bucks and my 400mm becomes a 800mm? yes please! So like any good internet buyer, I started doing my research. Reading manuals and reviews, checking comments/ratings on sales sites, etc…

During my research I read some reviews discussing the products it will work with, including complaints about the 100-400mm auto focus not working. It turns out, not all “L” lenses are created equal. Canon Autofocus systems will not focus with an Aperture of f/8 or smaller, the 100-400mm becomes a f/11 when the 2x Teleconvertor is added…. thus pushing it out of parameters. There were some comments that said Live View Autofocus works, possibly because of using phase shift to determine focus.

So I tried searching for more information, but the results are hit or miss on details. Some say it works, some say it doesn’t work. So I decided the only way to find out is to try it myself so I broke down and bought one. If it didn’t work, I would just return it.

So the purpose of this post is to give people an answer on what works and what doesn’t.

Technical Information: Canon 5D mark III, running Firmware 1.2.1, Canon 2x Teleconverter mark III, Canon EF 100-400mm f/5.6L, set to Single Center AF Point.

Facts:
1. Autofocus through the viewfinder DOES NOT work.
2. Autofocus through LIVE View, DOES work.
3. Manual Focus DOES work.
4. Image Stabilization DOES Work.
5. Image Quality (IQ) is lower, but hardly noticable.

So the good news is, the combonation does work in nearly every circumstance except regular autofocus.

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