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Last week I removed the front element from my Canon 28 - 135mm lens to remove a grain of sand from the lens. It was easy to remove the 4 screws, take of the top layer of glass and remove the grain.

I put the lens together again and I thought all was fine, wrong.

A week later I took the lens out for a shoot and the AF will not lock on, now the AF is constantly trying to focus and will not lock on to anything.

Have I broken the lens for good?

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    Possible answer could be here, I would treat it as options you can work with. – Peng Tuck Kwok Oct 11 '12 at 2:26
  • I'm sure you have just missed a "centering shim" or adjustment. The bad news is that sending it off to Canon Service will cost more than the lens is worth. Its not a bad lens, but I've seen the current price on Craigslist as about $200 in good to excellent condition. – Pat Farrell Jan 11 '13 at 20:58
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Were there any washers under the element? If there were, they should be placed back in the same spot.

Was there any other sand in the lens? It's possible more has gotten elsewhere within the lens during the process.

Does manual focus still work?

It probably just needs an adjustment. Take it to Canon to see what they can do and let them know what you've done to it.

The cost for this should be low as there is no dismantling, however it might be found that it needs further investigation and testing which can raise the cost.

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Your procedure should not have resulted in inability to focus lock if you did it properly. I'm wondering about the grain of sand, though. If one grain got in, likely there is more. This likely can be resolved by having the lens cleaned and calibrated. More of the lens than you dealt with will be dismantled, cleaned, and reassembled. This procedure is not for the faint of heart.

That's not a great lens. Maybe you should use this as an opportunity to get a better lens.

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I have Canon's 28-105 USM lens - and from a cleaning experience I know that underneath one of the elements is a plastic circular disc that is butted up against part of the lens - unscrewing something in the lens may have released something unseen, beneath the piece you were getting at. Picture it like a computer screw. You can unscrew a long screw from the side of the power supply to release it from the PC case, but, you might also, quite unintentionally unscrew the mounting screw for your fan at the same time. My guess is you'll have to go deeper into the lens to fasten something BELOW the lens you removed.

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