Using a modified version of Steve’s technique, I think the EX1R comes in at ISO 320. I used an 18% gray card, lit evenly, and adjusted my aperture so that the camera’s brightness meter read the card at 50%. Based on a spot reading of the card, and using the same aperture setting and frame rate as my camera, my trusty Sekonic 758DR reported an ISO of 320.
Has anyone else tried anything similar?