I recently bought an extender EF 1.4 III to use with my 100-400 L IS telephoto lens. My camera is an EOS 600D. The autofocus works well with the lens mounted on the camera. The autofocus also works well with the extender mounted with other lenses. The autofocus does not work with the combination of the extender and the 100-400 however. Is this a known problem related to the camera body maybe? Have someone encountered this problem, please advice. Regards Erik
Had a similar issue using a x2 Canon extender on a 7d mark ii body with 70-200 f2.8 - found that the lens/extender needs to be attached with the camera body turned of and the focus lock needs to be on the minimum focus range setting when in use otherwise it wont focus at the upper end of the lens! Hope this helps!
I may be saying what's obvious to everyone... I've noticed lenses become more difficult to focus beyond f/5.6, and even then when the light is not as bright f/5.6 can start hunting more often. Attaching a 2x multiplier reduces the amount of light gathered by the lens by 2-stops. This reduces an f/2.8 lenses to f/5.6. So depending on the lens, say an f/3.5-4.5 or f/4-5.6 variable aperture zoom, using a 2x multiplier reduces the light beyond f/5.6 from the get go. The other problem is using a cheaper 2x multiplier can produce lower contrast and resolution which makes the AF sensors have more difficulty locking focus... and that can be compounded by how "sharp" an image the original lens produces at it's wide open aperture. A "sharper" lens at wide-open aperture when zoomed to its longest focal length setting can foreseeably make it easier for the AF sensors to lock focus. This problem is one of the compelling reasons it's nice to have a quality 1.4x multiplier in you camera bag! It only reduces the light loss overhead by an extra stop from the lens' original f/#, and it typically preserves contrast and resolution better than the 2x multiplier which makes it easier for AF sensors to find focus.
My suggestion will not fix the auto focus problem but turn the auto focus off and manually focus at least it will stop the1.4 and 2x extenders giving you grief and you can still get your shot in, that's what they used to do before auto focus become popular and what I have to do with my 120-600mm Vivitar fd mount lens. The next thing I would suggest is raise your ISO up to 400 to 800 or more, my reasoning for this is when you use the 1.4 extender you loose 1 stop of light and with the 2 x extender you loose 2 stops of light. Th 400 mm lens needs a lot of light an with the extenders there is less light hitting the sensor so the camera can not auto-focus properly so increase the sensitivity of the camera by increasing the ISO.
