We didin't touch the MSI until you told us. The problem appeared BEFORE (with default 20 MSI value) ... It only happens in a specific interior view as the others cams (interior shots included) gave us results we were expecting. We already have done a lot of images with Corona without any problem - this is why we would like to understand what's really happening.
If "the problem" is what is described in the article I have linked earlier, then it is, again, EXPECTED to happen with the default value of MSI 20 an it will NOT happen with MSI 0.
However, I am not sure what the issue here is, so I am just guessing.
In this particular case, it looks like the exposure control is virtually locked and we couldn't succeed with 1.3 / Daily builds / non-RC neither. Adding a portal resolve the thing a little bit (we gain like 1-2 exposure levels) but the image deseperatly looks dark !
I am not sure if we are talking about the same renderer here. ;)
Portals should never introduce any exposure changes in Corona. In fact, they should introduce no changes at all, except for sampling (noise, image quality).
In a second time, AFTER reading the article you gave us, we tried changing the MSI to 0.
That would confirm that this really is an expected behavior.
So, the new question is : is that normal having to play - when the light is rare - with two parameters (MSI + EV) instead of a single one (EV) or is that a problem ?
Changing EV AFTER an image is rendered may sometimes produce different results than changing EV BEFORE the image is rendered, as described in that article.