Zooming my 15-year-old EF 100-400 L push/pull lens had become quite stiff. I did some research and found lots of info about zooming becoming too loose on these lenses but very little about fixing zooming that had become too tight. One poster mentioned that he had repaired his lens by removing the ball bearings that had come loose inside the Clamp Ring and now his zoom was smooth once again. I took my lens into a repair shop and they told me that Canon no longer works on this lens and quoted $125 to $400 for their shop to repair it. Being a cheap skate I decided to tackle the job myself. I found two Youtube videos on how to remove the Focusing Ring but nothing about the internals of the Clamp Ring. The official Canon parts list for this lens shows no detail regarding the internal parts. Given the difficulty of herding dozens of tiny ball bearings into two thin bearing races I can understand why they would sell the Focusing Ring only as an assembly. I found several Focusing Ring assemblies on ebay for about $200 if you don’t want to abandon the ball bearings. Tools needed: #0 phillips screw driver A tiny flat blade screwdriver (3/64”) A magnet to round up all the tiny ball bearings and screws. I suggest magnetizing your screwdrivers by swiping them over a magnet a dozen times or so. The screws are tiny and trying to reinstall them just using your fingers is exceedingly awkward. With magnetized screwdrivers you can use the driver to pick up the screws and guide them into their respective holes fairly easily. I also suggest working on a light-colored surface with lots of light so you can see the screws and bearings you’ll inevitably drop. A magnetic parts dish is also great for organizing the screws and herding the ball bearings. In this how-to I’m going to remove the ball bearings completely. Time to do the deed: Step 1 – Use the flat blade screwdriver to remove three tiny black screws in the chrome Name Ring at the front of the lens (Canon part# YA2-3647). Remove the chrome ring and the red plastic ring. You can leave your lens cap in place when disassembling. Step 2 – Use the Phillips screw driver to remove three screws at the top of the top Zooming Ring (YA2-3629) then slide it off the lens. One of these screws on my lens refused to cooperate with me and I had to drill it out. So far the lens works fine with only two screws. Step 3 – Remove a small Philips screw and a brass washer from the long slot in the side of the Focusing Ring (CY1-2833) then loosen the Clamp Ring and slide the Focusing Ring off the lens. Note: As I slipped my Focusing Ring off I heard several ball bearings hit my work bench. When I looked inside the ring I could see many more in the gap at the base of the ring. Note: If your lens has a LOOSE Clamp Ring, now is the time to repair the felt-like coating on the friction ring. No need to disassemble the lens any further. Basically, you remove the old felt-like material from the Friction Ring and replace it with a layer or two of painters tape. Check Youtube for instructions. Step 4 – Remove two white Phillips screws from the Clamp Ring then unscrew the Clamp Ring from the Focusing Ring. Be very careful not to spill the internals of the Clamp Ring as there are 4 metal rings and dozens of tiny ball bearings in the base of the ring. Carefully remove these parts one a time while noting how to re-assemble them later. Gotcha: I managed to dump out the 4 parts without noting how they went back in. Boo, hiss. I used the magnet to collect all the ball bearings and saved them in a small zip-lock bag in the unlikely case I might find some reasonable way to re-install them at some point. The Canon parts list for the lens is notably lacking in any clue how to reassemble them inside the Clamp Ring. Youtube and the Internet were also lacking so I put them back together a few times until it seemed right. Re-assemble the lens in reverse order. When re-installing the brass washer and screw in the Focusing Ring slot make sure the cupped side of the washer is facing up. When the Focusing Ring is back in place, try tightening and loosening the Clamp Ring making sure that zooming is as you like it when loose and that it locks the lens when tight. If it works properly, button up the lens and you’re done! Three soccer games later my lens zooms smoothly and the clamp ring keeps the lens from extending when the camera is held vertically.
