# 100-400mm will not stop down



## Mharwood16 (Oct 13, 2014)

Hi all, 
Ok so my mom has had a canon 100-400mm for several years now. I was playing around with it the other day and noticed something strange. The photos were looking very bright for being around f/16. I check to make sure I was on manuel and that Auto I SO was off on my 70d. The same results happened. It appeared as though the camera was thrilling me that the lens was stopped down but it actually wasn't. Thingng that I was doing something wrong, I left the lens off my camera for some time and then returned to it. No luck. It still wouldn't stop down. So the next day I tried it again of a different camera, this time a 7d. No luck again. The pictures have the same exposure weather the f-stop is at f/5.6 or f/40. I'm planning on taking it into my local camera store to have the repair department look at it. But I was wondering if any of you guys had any experience or suggestions.
Thanks 
Mharwood 16


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## rs (Oct 13, 2014)

Try stopping down the lens, and then pressing the DoF preview button. If the aperture blades do work, the image in the viewfinder should get darker. You can also look down the barrel of the lens and see the aperture blades while that button is pressed (again, if they're working). You can confirm how this should work by trying it with a working lens. 

If they're definitely not working, it could be worth cleaning the electronic contacts on the lens, but failing that, I'd suggest a return trip to a canon service centre for that lens.


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## Mt Spokane Photography (Oct 13, 2014)

rs said:


> Try stopping down the lens, and then pressing the DoF preview button. If the aperture blades do work, the image in the viewfinder should get darker. You can also look down the barrel of the lens and see the aperture blades while that button is pressed (again, if they're working). You can confirm how this should work by trying it with a working lens.
> 
> If they're definitely not working, it could be worth cleaning the electronic contacts on the lens, but failing that, I'd suggest a return trip to a canon service centre for that lens.


 
+1, its easy to do a check.

The camera gets feedback from the lens about the aperture responding to commands, and gives a error if it does not feedback the requested aperture, so look at it, set it wide open, then reduce the aperture and press the DOF button. It will be a obvious change. Cleaning contacts is all you can do.


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## Mharwood16 (Oct 14, 2014)

thanks guys! ill try that when i get a chance.


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