EOS 90D & EF 100-400mm - Auto Focus doesn't work with 2x extender

[jcampbriar]

I have a 90D with an EF 100-400 lens.  When I put on my 2X extender the autofocus doesn't work at all.  Is there some setting I need to change?

[normadel]

This IS a simple matter to discuss. The manual for this lens very clearly states that AF does not work with extenders 1.4X or 2X.  Manual Focus must be used.

See for yourself, on page 17.....

https://gdlp01.c-wss.com/gds/6/0300017486/01/ef100-400f45-56lisiiusm-im-e.pdf

You really have to consult the manual for a lens to see if it is functional, and how, with extenders. Some lenses forbid it entirely, or AF won't work with aperture smaller than f/8, etc. 

[johnrmoyer]

For any camera that has DPAF in live view, then the autofocus will work in live view no matter what F number so long as there is enough contrast and enough light. But you are correct that autofocus will not work when using the viewfinder. Also, bright light and strong contrast is needed. The manual also says auto focus for my EOS 80D will not work at F/11, but that is referring to when the viewfinder is used. Many things work that the manual says will not, but they do not work in all circumstances. A Canon extender will not work with this EF70-300mm f/4-5.6 IS II USM lens because there is not enough space behind the last lens element, but this particular Kenko teleconverter does work. Also the Canon 2x III with the 100-400 will work.

Autofocused in live view at F/11 with a combination of lens and 2x extender that the manual says will not work, 600 mm , Canon EF70-300mm f/4-5.6 IS II USM, Kenko Teleplus HD C-AF 2X DGX teleconverter