I have RD Exposure and use it to determine which of my bracketed exposures are the best in terms of ETTR. I have a Sigma DP2M and shoot at ISO 100. I believe It's telling me which of the 3 exposures is closest ETTR. I'm now considering using higher ISOs for B&W as I've noticed the camera works well at ISO 800 for B&W. After doing some tests it appears that I can't use RD for determining the best bracket at ISO 800 (or any higher than 100).
Here is what I'm finding. If I expose at f4.0, 1/60 sec at ISO 100 and also expose at f4.0, 1/500 at ISO 800 (these are equivalent) then RD shows just .5% pixels as over exposed for the ISO 100 image, yet the ISO 800 image shows as being considerably underexposed.
I'm not saying anything is wrong. I'm guessing that my expectation that I can use RD to assess which exposure is best at various ISOs is incorrect and that it cannot be used in this way….and I should just use RD to calibrate my camera. Exposures at ISO 100 have been basically ETTR. Just looking for confirmation of this, or if there is a way to judge which bracketed exposure is best.