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EF 400 2.8 Ver 1 lens and EF EOS R Adaptor very difficult to attach to EOS R5


I was fortunate to get this lens from My brother. When attaching to the adapter (R-5 camera body), the fit between the two was very tight and very difficult to attach. Almost felt like the threads were not aligned when attaching. Once attached, the camera appeared to not see the lens, no autofocus, and the F-stop on the camera body registered F00. After a couple more attempts, the lens with the camera body appeared to be working normally although very hard to attach. The lens was functioning normally prior to me receiving it and was used on my brothers R6 with the same adaptor with no issues.

Any thoughts



Greetings ,

I have a control ring adapter.  I've mounted a few EF lenses to it.  It's no different than mounting a lens, press and gently twist.  Significant resistance usually means misalignment or a damaged mount.  I would look at both sides. Make sure there's not a loose screw on the bayonet of the lens.  The bayonet needs to be pressed firmly and evenly against the mount before you start to twist.  It's easy not to engage the bottom of the flange evenly when you're holding a long lens in one hand and a body in the other.  Look closely at your technique and inspect both sides for abnormality.  If you notice a problem, contact Canon.  


Bay Area - CA

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Hi and welcome to the forum:
What brand is your EF-RF adapter?

cheers, TREVOR

"The Amount of Misery expands to fill the space available"
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
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" The lens was functioning normally prior to me receiving it and was used on my brothers R6 with the same adaptor with no issues."

That pretty much rules out the advice given so far. Some time, some where between him using it and you getting it, it had to receive some damage. I would go back to brother and see if it still works on his R6. The difference in an R6 to an R5 makes no difference. Plus the brand of adapter makes no difference since it worked before.

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



Thank you.  I retried the convertor on my current lenses and it is smooth.  Will look at the lens more closely for any issues. 


I have experienced a strange issue with an EF to EOS R mount adapter in the past, when I attached a specific lens to the adapter the camera rarely recognised the lens, and often showed the aperture as F00 on the display. I switch the mount adapter for another one and the problem went away completely. 

It's possible that with a big heavy lens on the mount adapter there might be extra strain that could cause it to raise a fault. 

EOS specialist trainer, photographer and author


Could be but it was pretty evenly supported at the time. I am going to an authorized Canon repair shop tomorrow and have them take a look at it. My suspicion is this lens was made back in 1991 and there is just the slightest difference in measurements in the mount on the lense to accommodate the converter.


My Canon control ring adapter was a pretty snug fit on the camera body the first few times I used it. 

"The right mouse button is your friend."