I ask because I recently tested the Blackmagic Ursa and my (possibly inaccurate) test revealed that the camera was shooting at around 50 ISO when set to 200 ISO. I know cameras often shoot slightly over or under what they say they do but this seemed a little too much and is making me think that my method of testing is completely wrong.
Essentially, I set up the camera about 2 metres away from my grey card and put on an 85mm so that the card filled the majority of the frame. I then lit the card to a flat stop and took a reading on my meter at 200 ISO and set my stop accordingly. Once in Resolve, I pulled up the waveform and the horizontal line of my grey card fell well below the mid-grey threshold. I re-did this test until my grey card was falling on the mid-grey line in the waveform, and by that point my meter was reading about 50 ISO.
For the record my lenses measure in T-stops, I wasn't using anything like a speedbooster and all the settings on my meter were the same as the camera.
To me this seems like a sensible way to test that sort of thing but I could be totally wrong and if I am, does anyone have a better/more accurate way of testing this?