Most light sources illuminate 4 tiles (20 feet) in all directions. Below is a list of common light sources.
TORCH : 4 tiles (20 feet)
GLOW POTION : 4 tiles (20 feet) away from the LIFEFORM
SHIMMER SHELL : 4 tiles (20 feet)
How a character sees can change during the game. It depends on their position, their EQUIPMENT, and the weather. Below is a list of some situations and how they affect visibility.
CLEAR CONDITIONS : See up to 1 mile
FOG : See up to 9 tiles (45 feet)
RAIN / SNOW : See up to 18 tiles (90 feet)
HEIGHT (20 feet or higher from the ground) : See twice as far
USE A SPYGLASS : See twice the detail and twice as far