Update: the Cooke 5/i lenses have an external dimmer, made by Transvideo. As an alternative to using the iris ring to control the focus scale illumination settings, you can plug one end of this small box into the camera’s RS receptacle, and the other end attaches to the /i connector on the lens. Voilà. Brightness is controlled by pushing the buttons, and you can also independently illuminate the scales on lens left and lens right sides.