Wear and tear on contacts

My 300 mm f2.8 lens has done 8.5 years hard work, almost invariably with a x2 Mk II extender attached. Initially I dismantled re-attached the extender every time I used the lens until, about 2 years ago I started to get problems with the extender and lens not coupling tight enough. After that I left the two coupled almost permanently. Now play has developed between the two and autofocus is beginning to fail once or twice every time I go out. It wasn't until I photographed the contacts on the lens and blew the images up that I discovered, to my horror, the extent of wear and tear on the lens. Has anyone else had this experience? If I replace the lens and extender, how can I guard against this in the future?

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I assume the contacts and mount can be replaced. I assume it is a common repair on older lenses and is probably not terribly expensive.

... I assume ...

Yeah, these are all replaceable parts which you could replace yourself or have any service center do it for you.

I don't think there's anything you can do to prevent it from eventually happening again over time. Mounts with less movement between them would likely see less wear over time than ones with more movement.

I have just heard that the new version of the 300 mm f2.8 (Mk II) is considerably lighter than the old. (I have just weighed my Mk I lens on pretty accurate kitchen scales, without hood and collar, and it is about 2.635 Kg) The advertised weight of the new version, naked, is 2.35 Kg. A very significant difference.
My only experience, in Australia, with sending stuff to Canon for repairs is that they sat on it for 6 weeks, at the end of which time they "haven't had time to look at it yet". The prospect of being months, rather than days without my lens is not a pretty one, especially in the middle of the nesting season.

You can definitely replace the mount (silver part) yourself, replacing the gold contacts may or may not require soldering. Replacing the mount is fairly straight-forward, I have replaced one lens mount myself. You will want a decent set of Jeweler's screwdrivers.

Here are some parts catalogs from Canon, not sure with 300mm f/2.8 you have:
https://photography-on-the.net/forum/showthre​ad.php?t=356679

This is the one for the 300mm f/2.8L IS (version 1):
http://www.f20c.com …%202.8%20L%20IS​%20USM.PDF

The mount costs around ~$30 from Canon, but you will want to measure the thickness with a caliper as they use various thickness mounts even for the same lens. Edit: Looks like they may actually only use one type of mount for this 300 2.8 IS.

Have you tried cleaning the contacts and tightening the lens mount ?
A bit of metho on a earbud will clean the contacts (do the lens, both sides of the TC and the body, make sure it's not too wet though, I dip the earbud, then dab it on a tissue so it's wet but not dripping)
And tighten the screws on the mounts, grab a 000 size screwdriver, and make sure they are nice and snug - try and do them as opposing pairs too, so they tighten down evenly (like you would tighen a cars wheelnuts)

This may fix your issues, or at least prolong their life till they need a service.


As for avoiding the wear and tear - it will happen every time you mount and dismount the lens.


And for what it's worth, some of the lenses we have at work have Bayonets in much worse shape than yours, and they work fine (they don't have that brass groove mark from the locking pin though - the damage on ours is from people not using lens caps) EDIT - had a look tonight, it seems a few lenses have been back to canon, they all seem to have nice shiny new mounts

Thank you all for your hints. After a bit of experimentation in the back yard it seems the extender, not the lens, may be the problem. The lens worked OK without the extender for about half an hour, but 300 mm alone is just too short for wildlife.
I have given the points at both ends of the extender a good clean, tried to stabilise the lens/extender joint a bit, and took a few more shots in the backyard OK. It just needs to have a proper trial in the field now.
Not game enough to start putting in spare parts myself. I have a biological background and am hopeless with mechanical things.