Image stabilisation disabled 5D mk3

I'd appreciate help with an odd problem.

Recently purchased a Sigma 150-600 C lens. The OS works brilliantly when mounted to a 7D but not at all with 5D mk3.

Is there a setting I may have inadvertantly changed on the 5D which would disable a lenses OS?

Edit - I have just tried another lens with stabilisation with the same result. No OS on 5D.

Just dug out my old Canon 70-300 to try.

The IS works fine on that with the 5D but not on either of my Sigma lenses.

Sounds like a compatibility issue. Have you any friends with different lS lenses you can try? My guess is it will work on the Canon ones, but not third party ones like Siggies or Tammies. You'll probably need both camera and lens sending to Sigma. BTW, how do you know IS isn't working? Did you get an error message?

I know it isn't working because when mounted to 5D the image through viewfinder is the same whether OS switch on lens is on or off. I have tried all OS options using USB dock. I also tried setting DoF preview button to 'IS Start' but no there is no audible or visible sign anything happens.

When mounted to 7D there is a little judder before the image stabilises brilliantly. You can hear,feel and see the OS initiate. The difference between bodies is dramatic.

I have read reviews of the 150-600 being tested on 5D Mk3 which say OS provides 3 stops of stabilisation. It gives none on my body.

I did try a Canon lens with IS as mentioned which worked fine.

Gungnir wrote in post #17628623
Edit - I have just tried another lens with stabilisation with the same result. No OS on 5D.

Long shot but try reflashing the firmware. Two lenses not working properly on same body would make me suspucious of the camera.

What was the 2nd test lens?

Canon lens with IS works. 2x Sigma lenses with IS don't. sounds like a comparability issue. Have you checked with Sigman re comparability with 5DIII. Are you running the latest 5DIII firmware as well as latest Sigma firmware? Did you remember to to burn the entrails of a chicken and smear it's blood on your forehead before starting testing?

By the way, at least the Sigma 150-600 C lens you initially mentioned has a firmware update from early 2015. Are you running the latest firmware? You also need the latest dock software (1.2) to do these updates. More info at http://sigma-global.com/download/en​/information.html.