A Path to Steam
Your eyes adapt rapidly to darkness. Each level of this ability (maximum nine levels) allows you to ignore -1 in combat or vision penalties due to darkness, provided there is at least some light.
Example: Night Vision 4 would completely eliminate darkness penalties up to -4, and would reduce a penalty of -7 to only -3.
Regardless of level, Night Vision only works in partial darkness. It has no effect on the -10 for total darkness (for that, get Dark Vision, p. 47).v