You can’t adjust FOV and Res without changing Pixels per Degree as well (PPD).
The Mask instead is making the rendering discarding the pixels from the pixel shader. The smaller the mask, the less pixels to effectively render. However, the FOV setting isn’t reducing the mask, it is reducing the FOV only, therefore, the game renders a narrower view but still within the same mask, therefore smaller FOV with more pixels per degree, therefore higher def. I can clearly see the difference in resolution in the HMD between 50% and 100% FOV.
Now this FOV experiment is showing a side-effect validating my other topic:
[BUG/FEATURE] Implement a metering system better suited for VR
I’ll come back later on this other topics with my findings.