Baseline (Widescreen) Solution & Issues
By default the FOV is Vert- (although the baseline is probably 16:9) and the Vert loss can be compensated using the FOV slider under Gameplay Settings.
There is however a better method to ensure Hor+ screen change across the board:
Open Documents\my games\Painkiller Hell and Damnation\PKHDGame\Config\PKHDEngine.ini
Look for the line...
AspectRatioAxisConstraint=AspectRatio_MaintainXFOV
And change it to...
AspectRatioAxisConstraint=AspectRatio_MaintainYFOV
Now run the game and lower in-game FOV to 63, regardless of your aspect ratio. This is the newly-fixed Vert FOV, now applicable to all aspects.
(YFOV 63 = XFOV 95 in 16:9, which assumedly is the baseline. Feel free to fine-tune according to your personal taste.)
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Same as Multi-Monitor Solution:
Open Documents\my games\Painkiller Hell and Damnation\PKHDGame\Config\PKHDEngine.ini
Look for the line...
AspectRatioAxisConstraint=AspectRatio_MaintainXFOV
And change it to...
AspectRatioAxisConstraint=AspectRatio_MaintainYFOV
Now run the game and lower in-game FOV to 63, regardless of your aspect ratio.
Alternative FIX for the BlackBars in Menu and Cutscenes --> Hex edit this File: ...\steamapps\common\Painkiller Hell & Damnation\PKHDGame\CookedPCConsole\Engine.upk
Look for 4C 8E E3 3F and change to CD 90 18 40 and in the same Line 00 00 B4 42 to 00 00 F2 42
I have this File ready in the files available on PC Gaming Wiki --> PKHD_21_9.zip
Eyefinity / Surround Specific Solution & Issues
Same method.
Actually the manual edit is the only way to get a satisfactorily Hor+ FOV in multi-mon: max XFOV is 120 which is not enough even for 15:4. Using a fixed YFOV allows for higher XFOVs.
Held weapons are always Vert-, to the point of hurting gameplay in multi-mon.
Scope sights overlay is a 16:9 design that is squished in lower ARs and stretched in multi-mon.
4k UHD Specific Solution & Issues