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Focus failure, EF 100-400 f/4


I was shooting with this lens on an R7 body, with adapter. No problems until it stopped focusing with Autofocus entirely. I switched to manual focus with no luck. I tried changing to a 5D III body with similar results. Only able to focus with either manual or Autofocus at about 120mm, 250mm, and 310mm; all other focal lengths will not focus in either focus mode. I've "cleaned" the contacts and switches by flicking the switches repeatedly by hand, and dismounting and mounting the lens repeatedly. This lens dates to around 2017 and has been serviced once by Canon Professional Services. Is there any "fix" for this or is it just time to return it to CPS for servicing




With the lens off the body, zoom in and out fully a few times.  Then turn the focus ring a few times in each direction.  Now return to 100mm.

Remount the lens and retest.  I do not expect this fix resolve the behavior, but its worth trying.  Also, even if it works again, it could stop functioning again if there is a faulty component.  

Give it a try and let us know.  

Bay Area - CA

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Thanks for your suggestion, Rick, but this had no effect. I'll renew my CPS account and send it in for service.


"... is it just time to return it to CPS for servicing"

Yes it is.

You can try all that stuff like cleaning contacts and mess with switches but it needs to go to Canon for service.


"... switches by flicking the switches repeatedly by hand ..."

No! I would stop doing that. All you are accomplishing is wearing out the switch faster

EOS 1DX and 1D Mk IV and less lenses then before!

Since I haven't moved the switches in years, I doubt that flicking them ten times would wear out the switches. At least I would hope that Canon would use durable components, LOL.


The point was not that you would actually wear out the switch, it was that it wouldn't do any help.

EOS 1DX and 1D Mk IV and less lenses then before!
click here to view the press release