Knowledgeable 5Dsr (or any camera) users help please.

Hello.

Does the 5Dsr have a base iso.
I have always assumed this to be 100 as less than that becomes LOW.

The tech guys where I work suggest it may be 160.

Any ideas?

The base is 100. Any other intermediate ISO is a push/pull after the fact. If your question is what low ISO provides the best result, that is a different question, but you are still relying on the camera to push/pull for you at 1/3 stops.

From the years of testing however, it makes little difference. Just shoot for proper exposure if possible at the lowest ISO setting you can go, and either process yourself during post, or let the camera process for you in-camera.