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EF 100-400mm f/4.5-5.6L IS II USM Lens switches to MF mid shoot


Hi all,

I run an R6, with EF to RF Converter, 1.4x iii Extender and an EF 100-400mm ii. I purchased from MPB back in April 2023. It has worked fantastically for birding, but just recently I'm in the middle of a session and the lens makes a metallic noise then switches to MF, from Eye Tracking AF? It's  normally after holding the lense facing down for a short period before swinging back up. The noise is like a gentle 'rattle' but only once and it makes no further sounds after, just when the switch happens. I am normally fully extended on lens too.

Its not settings as I haven't changed them. Buttons in the side of lens are AF, Infinity, Stabilisation on, Mode 1.

It focusses ok when in AF, so not sure whats happening? It's out of 6 month warranty with MPB now. I'm UK based.




This doesn't sound normal.  The attitude of the lens should not affect AF/MF operations. 

Is your adapter Canon brand?  What happens if you remove the TC, and just use the lens with the adapter, lens fully extended, pointing down?  Can you reproduce the behavior?  

I'd consider sending the lens to Canon for inspection.  

Bay Area - CA

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Good call  didn't even think of taking extender off. Will try that now. The lens doesn't rattle normally when closed up, but I'll try and replicate.

Ok, just took it off the camera to remove the extender and the autofocus element just at the end of the glass was wobbling a lot. No power going through obviously at that time. Put it back on the body and now I'm struggling to replicate. I'm lost. Other half has same set up and hers doesn't do it. There's no issue with autofocus when it's on and being used, so not sure why it's happening. Think a trip to repair shop will be necessary if it happens again. I'm no expert in lenses, but could it be loose or maybe a motor issue?


Check the programming of the Control Ring on your mount adapter, if it has one. 

"The right mouse button is your friend."

It's set to exposure control as normal. Noise is from further up lens when it happens. Thanks for that check though...

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