Canon 500mm f4 II trouble

Hello,

About two weeks ago, I started getting overexposed photographs on my rig (1dm4+500mm.) Way overexposed. First I thought it was the shutter, but after much troubleshooting and checking,
I have nailed it down to the lens.

It will not stop down properly. it works fine at f4.

For example, if I meter and shoot at f/4+ 1/4000 sec it comes out fine. Then for more DOF I shoot at f/5.6 + 1/2000 - and it is overexposed. Then I use f/8 + 1/1000 and it is WAY
overexposed. And so on... (similar results across 1dm4, 7dm2 and 6d)

Has anyone encountered this problem - is it a common one? I am going to have it looked at tomorrow. But repairing will probably entail sending it all the way to Singapore (I am in
Dhaka Bangladesh with limited Canon repair facilities.) and a long wait not to mention $$.

Are there long lead times on parts of this lens? (I noticed a diaphragm assembly in the partslist, but at this point it is too early for conjectures...)

It is out of warranty and I have shot perhaps 20000 pictures with it in the last 2.5 years.

If anyone has encountered this problem please let me know.

Many thanks,
Ihtisham

I am not sure what type of testing you have done, but did you try the lens on another body, or if you only have one body did you try another lens on it? It sounds like the aperture is stuck wide open. When you dial in, say, f/8, is that what it shows in your display? Does the lens stop down if you press the dof preview button? Make sure the thumbwheel "lock" is off too.

Hi Daniel,

Yes, I tried on several bodies with the same result.

Lens does not stop down when I press the dof preview button on the 1d mark IV (that button works because my 24-105 stops down.)

What is the thumbwheel "lock"? Is it on the lens?

Thanks,
Ihtisham

It's on the body, directly below the "wheel". You can lock it to prevent accidentally changing the aperture. But since your other lens works that rules that out. I'm afraid it sounds like a trip to Canon, unless there is something else being overlooked.

I was afraid of that....

Have you tried to clean the contacts?

Yes, I did - no luck. I am finding as I look into the lens and change the aperture with the dof preview IN, the aperture does not close properly.
Sigh.

I had the exact same problem and had it repaired by Canon. Total cost was $500 but paid $350 with CPS Platinum discount.

Thanks very much, Mike, for the info - it is very helpful as I was wondering (read: dreading) how expensive this would turn out to be. Ihtisham