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Canon EF 24 - 70mm L lens aperture not working - how to test the reason



I have an EF 24 - 70 mm L lens where the aperture has stopped working. If I look through the lens I see it is fully stopped down. It is not the body as if I use a different lens everything is fine.

Looking online there seam to be a number of reasons why the lens is not working including an issue with the aperture flex cable. the aperture motor and the aperture blades.

What I would like to know is if there is a way to use a multi-meter to check via the lens contacts if the problem is with the flex cable, aperture motor or something else. I can then source the parts required and do a repair.

I know that if the flex cable is faulty the resistance of the motor will / may not be available but if I know what resistance I should have between the various lens contacts I should be able to decide if I need to order parts or if the problem is just a matter of cleaning the aperture blades. I would like to strip, repair and re assemble the lens at the same time and not strip and then wait for parts before doing a reassembly.






Greetings ,

I've never seen any documentation for this, but it may exist somewhere.  In these situations, my recommendation is to send it to Canon.  If the lens is still under support It might be less expensive than you may think.  You will also be assured the lens will repaired quickly and operate to factory specifications.  A repair with estimate can be initiated through MyCanon.


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My  thoughts are similar to Rick's. If you have to ask you are probably not the person to do the repair. Sending it to Canon is by far the best course of action. 1 (800) 652-2666  At any rate that is where you need to ask any questions about any repair or repair specs.

EOS 1DX and 1D Mk IV and less lenses then before!


Thank you Shadowsports and ebiggs1 for your suggestions.

I will try following up with Canon and see what they say first. Subject to my response from Canon, I may do the repair myself. If it is just a matter of replacing a flex cable this would be within my skill set.

As I have the technical skills required to strip and re-assemble the lens I will go down this route and find out what is causing the problem and hopefully fix.

I found a Youtube video of how to replace the flex cable (that appears to be a common fault on this lens) so I may be able to trace the aperture motor connections back to the lens contacts and see if first I get some sensible readings from a multi-meter. I was hoping to find out if I needed to order parts before stripping the lens but it does not look as if this is the case.


The contacts on EF lenses are for digital communication. There's a micro chip inside the lens that distills the commands sent over the communication channel and assigns them to the various lens modules like AF, aperture control etc. 

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