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EF300 2.8 IS II, knob fell off mount+lost. Help pls It's not available as a spare part!


Does anyone have a spare Super Telephoto 'mount ring tightening knob' please?? The big knurled knob that you tighten to stops the lens rotating through 360 degrees when it is on the tripod.


Mine literally fell off my EF300 2.8 IS II lens whilst I was out shooting.


I was very surprised to be told by Canon Parts that they do not offer that big knurled knob as a spare part.


Instead they want me to send in the lens !  and have a whole new mount ring fitted !! for approx $270 minimum charge !!!


Surely if the mount ring tightening knob can fall off, it should be

a) available as a spare part (it's not)

b) replaceable without having to send the lens in - I use that lens every day!

c) fixable without having to replace the entire mount ring assembly


In the meantime, I am resorting to Duct Tape. 

The bonus of that is it makes my lens uniquely identifiable, and ?less likely to be stolen? 😉


Any help appreciated! Thanks




I recommend that you contact the product specialists at B&H Photo Video online. 


Navigate to the product page for your lens, and click the box to start a chat.  It will probably be in the lower right corner in most web browsers.  This will likely put you in touch with a person who I am going to call a "router."  Describe your issue to the router, and that person will connect you to a product specialist.  Many times the product specialist may actually work for the manufacturer.  


I recently noticed that I needed some screws for the Arca-Swiss mounting plate for my lens foot.  I think I swapped them out, and never swapped them back.  No matter, I wanted to oorder spares.  The entire chat took about 3 minutes.

"The right mouse button is your friend."


"!! for approx $270 minimum charge !!!"


Expensive lenses are expensive to repair.

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

To clarify

My issue is not that 'replacing the entire mount ring' as a repair is expensive. 


My issue is that replacing the entire mount ring is totally unecessary in this instance

The "repair" required is to simply replace the tightening screw that fell off, which is why I am looking for one


It would be very straighforward to put a new one on.. less than one minute probably,

and no expertise required other than the correct screwdriver, and which direction to turn a screw.


Every Super Telephophoto lens has that same screw.

If they can fall off, then they really should be stocked as a spare part


I am not wanting to pay $100's just to get a new screw


Hopefully someone out there will have a spare tightening screw for a Canon Super Telephoto lens? EG someone who broke a lens and did not get it repaired, or properly broke a mount ring?

Or a repair facility that has one in a bin on the shelf?


Thanks all


While I do simpathize, both Wadizzle and ebiggs1 have good suggestions and points. 


When I first saw the price for a replacement collar on the 70-200 I though you have got to be kidding.  I didn't lose one, I just happen to see them on B&H's website.  300mm is also expensive apparently.  Definitely something you'll want to hold on to.  


What about a knob from a knock-off, or finding a bolt with the correct thread pitch at a hardware store?  The duct tape thing...  that would be a no for me.  There is nothing that could get me to put tape on one of my lenses.  I hope you can find a way to replace it reasonably.   

Bay Area - CA

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"What about a knob from a knock-off, or finding a bolt with the correct thread pitch at a hardware store?"


This may be possible but if I remember correctly, I don't have that lens but I have a very good friend that does, that knob does not screw off.  So, something is broken and that is likely why the knob is not available separately.

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

Correct, it does not look to be just a simple screw, but some sort of assembly that has probably had a retaining screw to keep the big knurled knob/screw from falling off, so a trip to the hardware store is not on the cards unfortunately.


Thks for the thoughts

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