Question about Canon 75-300mm f/4-5.6 III USM EF Lens

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Canon EF 75-300mm f/4-5.6 Repair Manual

The apeture is stuck closed. Maybe at f/22. How can I get into this lens to repair it? Is there a free online repair manual?

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  • Anonymous Mar 31, 2008

    Iris or diaphram does not open

  • Anonymous May 06, 2008

    When trying to use lens on AF it will not focus correctly and when trying to focus manual it will not go back past 100mm

  • soony May 14, 2008

    Aperture is stuck on 80mm and can't be turn either way? How to i fix it??

  • photomom15 Jun 03, 2008

    Aperture is stuck (almost closed) on cancon 75-300 mm lens. How can I repair this? Mona

  • Jerry Zhou Jun 07, 2008

    Mine had the same problem. The electric flexible wire that links to the moving aperture motor in the center is most likely broken, causing the aperture to stop reopen after a shot. this could be proved if you feel something touching inside when zoom. (that is caused by the loose flexible wire, if so do not zoom anymore, you could further damage the wire) It need to be replaced and this requires the complete disassmebly of the lens. I repaired mine myself by adding a thin wire, took me more than 10 hours in total (first tear it apert, pin point the problem, then think of a solution because I did not have the parts for repair, then finally repaired it). If you have never done lens/electronic repair before, don't do that, too many tricks that could lead to further damage.

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Mine had the same problem. The electric flexible wire that links to the moving aperture motor in the center is most likely broken, causing the aperture to stop reopen after a shot. this could be proved if you feel something touching inside when zoom. (that is caused by the loose flexible wire, if so do not zoom anymore, you could further damage the wire) It need to be replaced and this requires the complete disassmebly of the lens. I repaired mine myself by adding a thin wire, took me more than 10 hours in total (first tear it apert, pin point the problem, then think of a solution because I did not have the parts for repair, then finally repaired it). If you have never done lens/electronic repair before, don't do that, too many tricks that could lead to further damage.

Posted on Jun 07, 2008

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How to lens disassemble canon 50mm macro 2.5


Just one page of a repair manual from Canon. Just to ask yourself if you understand how to adjust a lens. If you can't make up anything from this, it is not worth buying a repair manual for this lens, because bringing it to a certified Canon repair shop will be cheaper. And the result 1000 times better.
Opening a lens is no problem. Putting it together so you can use it as sharp as it should be, needs special tools and special skills.
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This is a simple and relatively inexpensive repair. Take it to a reputal repairer.

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My canon EF-S 17-85 IS USM lens will cause an error 99 when in wide angles at 35mm or less


There are moving contacts inside the lens which communicate focal length data back to the body via a voltage level, and they have either become worn or dirty.

Either way, when you set the lens to 35mm or less the voltage which passes the information drops out and the camera signals the error 99 to inform you that the lens has stopped talking to the camera.

This is not a user fixable item. The lens must either be repaired or replaced and the problem will get worse if left unrepaired. Contact an official Canon Service centre in your country, you'll find that there is usually a fixed cost repair menu, and often if the fault is a known issue the repair might be free even if the lens warranty has expired.

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My 17-85 is no longer capable of focusing to infinity. It focuses short and medium distances with no problem, but I have to use manual focus to take long distance shots. Does the lens need to be replaced...


Depends on whether it is still in warranty. If so, check with Canon for warranty repair. If not, I'd guess it will not be cost effective to repair it, but a local (maybe one in your area?) Canon authorized shop could give you a better idea about whether to repair or replace.

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The flat printed cables inside the lens unfortunately develop small cracks over the years and this causes the lens to fail. The repair is NOT easy, you have to tear down the lens all the way to it's base components and replace the flex cable on the aperture diaphragm. The cable and the diaphragm are available online, but again it's not a simple repair and most people should not attempt it themselves.

Thomas Drayton
Owner Darntoothysam com

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I hate to re-invent the wheel (and I don't want to cut 'n paste someone else's hard work to present as my own) so please click here for a likely practical solution which usually works and for Canon's own suggestions on the error 99 fault.

If this has assisted you then please rate my answer; if not then add a comment to provide feedback and I'll attempt to provide an alternative solution.

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If your AF motor is hunting then the lens probably needs to be looked at. I wouldn't recommend this as something to do at home so you will need to either find a repair shop if it is out of warranty or return it to Canon if it is in warranty.

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Autofocus function will not work on Canon 17 40mm 1.4 EF lens


Since the problem is limited to this lens, I highly doubt that you would have to replace the cameras AF sensors.
Since you only have the problem with this lens, I am afraid that you will have to get the lens repaired. Is the lens also 3 years old? Did you get it with MACK extended warranty?
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I hope this helps, 
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