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Dismantling / repairing a Canon EF 90 - 300 mm Zoom lens 1:4.5-5.6


My favorite lens no longer fully extends and makes a terrible crunching noise. I mourn the loss of this lens as I have had some fantastic photos from it and I found it very useful.


Having emailed my local Australian Canon repair centre and getting a quote for $200 ex GST + Parts, I looked on eBay for a replacement. None was to be found. I did find a website listing the lens at $269 ex but they had no stock (And have not had any for ages).


I decided to pull apart the lens myself and repair it. In the end nothing was broken. A part had come loose. It was getting stuck in the gears and preventing the unit from fully extending. I did however manage to damage $5 in parts and after receiving these replacement, forgot to order s specific screw. After rebuilding the lens and using a screw from an old pair of sun glasses, I am back in business.


The image on the left, is the loose part. It guides the lens extension in and out. It is held in with pressure so I just pushed it in. The part on the right is the part I had to break. It is a piece of metal used to stop the lens being extended too far, popping the extension right out and likely falling to the floor. The screw that holds it in place was so tight, I ended up drilling it out and then removing the single piece of metal and replacing it. This is now a screw from a pair of sunglasses.



This brings me to my tools. 1 x set of Jewelers screwdrivers, 1 x EF lens cleaning cloth, a set of tweezers and a hand drill.

The hand drill was needed due to the screw I could not remove. The replacement parts from a local Australian parts supplier cost under $5. I also recommend a blower. I kept the lens clean with the EF cloth and a blower.


Why write this article?

Many people have written similar articles for other Canon Lens's. Whether it be to dismantle, to regrease the sliding mechanisms or to repair a cheap faulty unit purchased from eBay. None covered the EF 90 -300. I saved myself money repairing the lens myself and I had nothing to loose. The lens was failing. I also can no longer buy this lens on the open market.


This process can be very frustrating if you don't know where to start and what to do next. That's what this page is for.




Is this dangerous?

Yes. And scary. There are so many things you can damage. There is ESD damage to electronic parts, stripping gears on the motor, loosing springs and damaging the protective coating on the lens. Then there is the dust you can introduce into the system. Thin electronic plastic wire/circuit tracks and the process is very tricky. Canon have neatly hidden the screws and mechanisms that hold these units together, making it hard to work with and remember that these are precision devices.


This is not for the faint hearted. If the image below brings you out in a cold sweat, don't do this. (Photo taken with a Phone camera as my camera was out of action as my lens was in pieces).





I want to reference the following resources. They gave me the hints I needed to do this and the confidence. Might I add, my lens is now like new. No dust bunnies, no problems. It performs as normal.

The first resource you should download and print is the Canon Parts list (With exploded diagrams).


Starting with the diagrams on the left and working to the right:


Make sure the lens cap is ON. We want to prevent any lens damage from miss handling.


Remove the 4 base screws and two screws that hold the base against the lens contacts. Use a jewelers screwdriver to carefully lever off the AF/MF rectangle mount. Don't be tempted to remove the circuit board etc just yet. Now slide the ring that holds this AF/MF section off. Carefully use a jewelers screw driver to unhook the 4 plastic data cables. They unhook in different ways so study carefully first.


The ring with the 90-300 mm calibration on it, use a jewelers screw drive to lever this off, over the end of the barrel. There is no need to remember where the 90 - 300 mm markings lined up. We work this out when we reassemble it.


Under the ring you can see the little piece of metal that stops the lens opening too far. We need to extend the lens out so that you have free access to remove the stop. This stop has the screw that was stuck for me, I had to drill it out (Carefully). You will also see the gold colored contact strip that is screwed in. Be very careful not to damage this and unscrew, remove it. It can be easily damaged as it is very fragile. You can now remove the motors etc from the back of the lens. Before you get too carried away, note the additional gold contact strip that is now exposed. Remove this one. If you do not, the next step will destroy it. You now rotate the lens and extend it further than normal and you will have it pop apart as in the image in the last column at the right.


From here is smooth sailing to unhook and disassemble the rest. Be careful not to touch lenses. Especially as you will likely get gear grease on your hands. Do not introduce dust.


Repair/replace/reseat the faulty part and reassemble.




Reassembly can be tough. There are various little ridges in the plastic that need to line up and it is not immediately apparent how to reassemble. Line up the moldings and carefully reassemble. Once you have the inner lens tube lined up and gripping in the outer tube, you need to rotate it just right and then reattach the gold contact strips and insert the little metal stop. To insert the metal stop, the lens needs to be partially extended and the "tooth" on the stop needs to fit into a plastic grove. I can't really explain this in text. You really need to be doing this to make sense of it. I used the EF lens cleaner throughout this process to keep the lens clean and the blower to blow out dust.


You basically reassemble the same as you dissembled but in reverse. When you get to the 90-300 ring, you need to place the other ring you removed, on the lens, as it has a groove in it and only goes on one way. Once this is temporarily on, you can see where the 300 position should be.


Remove the second ring, reattach the first (lining it up with the 300 position) and then finish reassembling.


All done.


This was written 13/3/09. On 27/6/09 I thought all my good work was undone. Whilst having a photo session at a local wildlife park, the lens held at eye level on the back of a Canon 10D, came loose and landed on the leading edge of the front lens, down onto cement with a terrible cracking noise. It was a sickening sound. It fell at least 1.5 meters and the lens took the full impact.


On picking up the lens, it rattled. The lens had fully extended and made a crunching noise when retracted. It would only retract about 1 cm of the normal 5-8 cm that it would normally retract. The lens glass seemed fine but the body (Focus ring) was cracked right through. There were fine filaments of plastic hanging out near the front of the lens. Obviously I was fearing the worst. There was impact damage along the front edge. I removed the four rear screws and the whole thing just fell apart in my hands. With a lump in my throat I started pulling the lens apart further than ever before. I started finding small pieces of broken plastic that were falling out of the body of the lens. I reseated everything, did not bother looking for homes for all the small bits of plastic. I thought the lens was a write off and I had nothing to loose. These things are precision made. Surely I could not bring it back from the brink ?


I reassembled, tightened screws, cleaned the lens and tested it.


It worked. It still auto focuses, it still has a clear image. It is still my favorite lens. It is now cracked right through but the aperture curtains work flawlessly and I managed to avoid scratches on the lens.


Phew. Miracles can happen.


If you find yourself in this situation you have a few choices. 

  • Send the lens to Canon (Or your manufacturer) for repair (They will likely be uneconomical)

  • Buy the spare parts yourself and repair (My last full repair with parts cost les than $20)

  • Buy a replacement lens (This is a $500 lens and I can't buy it anymore, not even on eBay.


I decided to see how a repair would go, after all the lens was basically dead.


Here is a photo taken with the repaired lens.




Other photos I have taken are available to view at http://www.flickr.com/photos/mickyj_photos/








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