STALKER with the AMK mod runs very dark by default (or at least it did when I installed it) and the flashlight did basically nothing. I made some changes to the lighting settings as below. I don't remember what the defaults were when I installed, but as I recall, tone mapping (i.e. HDR) was switched off entirely which was the majority of the problem. There is already documentation out there on most of Stalker's console options, this just covers tone mapping specifically as some are undocumented and it took me a lot of playing with them to get good settings figured out.

r2_tonemap - Turns tonemapping on and off. With this off, dark areas are very dark and the flashlight does nothing. Set to on.

r2_tonemap_adaptation - Controls the speed at which your 'eyes' adapt. 10 gives almost instant adjustment which can look odd if you look at the sky and then at the ground at night. 0.1 makes adjustment very slow, as in barely noticable. I ran 0.4 so that adjustment was fairly fast but not an obvious jarring effect when going from very dark to very light or vice versa. When testing tone mapping settings you may want to set this to 10, as some options do not apply instantly, making their function non-obvious if you have slow adaptation set. You can set this as you see fit once the others are set.

r2_tonemap_amount - Seems to basically control the strength of the effect. Setting this to 0 is the same as turning tone mapping off. 1 is the maximum, and what I used.

r2_tonemap_lowlum - Seems to control the amount of darkness allowed. Setting this to 1 basically turns tone mapping off. The minimum value is 0.0001, which is what I used.

r2_tonemap_middlegray - This is the powerful one. This controls the 'balance' of the tone mapping. Set to 2 it basically gives you night vision, set to 0.1 it basically turns tone mapping off. I spent a lot of time on this value, as setting it too low undoes the benefit of tone mapping, rendering you blind at night/inside, but too high gives you too much of an advantage compared too the AI's ability to see in the dark. I ended up at 0.75.

The flashlight is a good acid test for the lighting. When you go into a dark area and turn it on, the light from it should look like a flashlight, not like a spotlight strapped to your chest, and not like the batteries are dead. You should also be able to see a little at night or inside without it, as the AI can. Finally, the sky seems to react oddly to tone mapping, it is always somewhat bright even at night. No amount of messing with the settings fixed this for me.

Summary:
r2_tonemap on
r2_tonemap_adaptation 0.4
r2_tonemap_amount 1.
r2_tonemap_lowlum 0.2
r2_tonemap_middlegray 1.