UPDATE: 6/4/2019
6/4/2019 - This issue will be fixed as of Negative Lab Pro 2.0. If you were using the v1.3.1 pre-release, you will need to replace the old vuescan profiles with the new ones included in the package.
Original response:
Just to close the loop on this… you are correct that Vuescan is using the entire cropped area to evaluate exposure lock values. I guess what I meant is that in most cases, if you are cropping for the image itself, it shouldn’t make much of a difference if you also include part of the frame. However, I would STRONGLY advise that you NOT try to lock exposure on just the unexposed film border… This will not result in a good image for conversion later by Negative Lab Pro.
For example, this is a RAW DNG done after locking exposure on just the image border, and using “image” media type. Notice that blue channel is clipping in the shadows (which will be the highlight region in the converted image:
Because there is clipping in the scan, there will be some issues with lost channel information during Negative Lab Pro conversion.
Here, it causes funky blue-color banding issues in the sky and on the shorts of the subject:
If we were to go back on the same scan, and lock the exposure on a crop on the exposed image itself, we end up with a much more usable file, with a histogram that isn’t clipping on either side (note: I also changed media to “Color Negative” on this scan, which helps blue channel gain).
That looks much better! Now when we convert, we won’t have any of that weird color banding in the highlights, because we’ve got plenty of info!
Ah! So much better!
Hope that helps!