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?

[tgo2002]

Sorry I didn't spell out 6D Mark II both times.  I have that and the 5D Mark III.

[deebatman316]

Both of those cameras CAN focus at F/8. I would check the setting called "Lens Drive When AF Impossible". To see if the camera will keep searching for focus instead of stopping.

[tgo2002]

No filters, no teleconverter/extender.  The max aperture on the lens is 5.6.  I thought maybe low light would make a difference, but it happened today in daylight.

[deebatman316]

What about the subject's contrast. Your camera relies on CONTRAST to work NOT how bright it is. No first party Canon EF or EF-S lens has EVER had an aperture SMALLER than F/5.6 early lenses had wider open apertures. They were not smaller than F/4.5 but once they figured out that an aperture of F/5.6 provided enough light and contrast to the AF system. That allowed them to make cheaper and smaller lenses.

[tgo2002 · Accepted solution]

"Lens drive when AF impossible" was set to "Stop."  I switched to "Continue" and it seems to be working inside tonight.  I will try it in daylight tomorrow before declaring victory, but that might have taken care of it.

For what it is worth, the 5D was set on "Continue."

[deebatman316]

Older Telephoto and Super Telephoto lenses would often get lost and hunt if the camera couldn't lock focus. I had that problem when borrowing my friend's dad original EF 300mm F/2.8L USM (released in 1987) lens while using it on my old EOS 40D. That was Canon's first lens to feature a Ring Type USM AF. It also utilized focus by wire. This was before Canon had designed a clutch override system found in newer Ring Type USM lenses. 

[jrhoffman75]

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@tgo2002 wrote:

"Lens drive when AF impossible" was set to "Stop."  I switched to "Continue" and it seems to be working inside tonight.  I will try it in daylight tomorrow before declaring victory, but that might have taken care of it.

For what it is worth, the 5D was set on "Continue."

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Whenever there are two devices that should function the same but don't it is important to look for differences. You found one. Are the AF modes the same? Typically a mountain range is an easy subject for One Shot AF with single center point. Good contrast between the mountain and the sky.