Autofocus problem EOS 6D Mark II and EF 100-400mm

[tgo2002]

I have recently-purchased 6D Mark II camera and EF 100-400mm 1:4.5-5.6 L IS II lens.  The past few days the lens will not autofocus at 400mm some of the time.  If I zoom out to 250mm, focus, then zoom back to 400mm, it will focus.  I can re-create the problem easily on the 6D, but on my older 5D Mark III the lens works perfectly.

Has anyone else experienced this?  Is it a problem with the camera or the lens?

[Waddizzle]

Check the focus limiter switch on the lens!  

Make sure you are not too close to subjects!

[tgo2002]

I'm pointing it at a mountain and it only happens on the 6D, not the 5D.  So that is not the problem.

[jrhoffman75]

If it works on one camera but not the other I would probably rule out the lens.

Be sure that the subject has sufficient contrast. The 5D Mark IV does have a greater number of AF points, so that could be relevant.

[deebatman316]

The 5D Mark III can also AF at F/8 which the original 6D can not. I would also check the setting called "lens drive when AF Impossible". It should be set to "Continue Focus Search". Also if filters are attached remove them. The EF 100-400mm F/4-5.6L IS USM lens lineup doesn't AF with them well. They cause problems like this all the time.

[jrhoffman75]

Valid points, but I wouldn’t expect them to be camera dependent , considering that there is no mention of a teleconverter. 

[deebatman316]

Now I really wasn't focusing on teleconverters or AF at F/8. But I was focusing the "lens drive when AF Impossible" setting. The 5D Mark III & IV use the same 61 point AF system. It kinda sounds like a communication problem though with the 6D.

Edit: The OP mentions BOTH the original 6D and 6D Mark II. So is the problem with both cameras?

[jrhoffman75]

The AF Drive setting is a good item to check. The longer zoom can enlarge the subject and make it harder to get contrast. But, since the OP says he is trying to photograph mountains that may reduce that possibility. 

[deebatman316]

Focus limiter switch as @Waddizzle (Bill) brought up? That could be it too. 

[jrhoffman75]

OP says mountains.