[DF] Light sources

[Kromm]

DF shoots for simplification, so yes, there's just an arbitrary cutoff. It isn't as extreme as you make out, though. The way it works:

A light usually eliminates darkness penalties to combat and vision, for everyone, in a semicircle in front of the bearer. Spells like Glow and Sunlight eliminate penalties over their entire area of effect. Vision and combat are possible at -3 out to triple these ranges.

Thus, a torch converts any darkness to no penalty out to 2 yards, converts penalties worse than -3 to -3 out to 6 yards, and gives no benefit beyond that. This is as complicated as DF cares to get.