Rules: Dragon Sight

As noted in the Monster Manual, dragons see extremely well in all lighting conditions.
In normal light, a dragon sees twice as well has a human. In game terms, this means that a dragon can detect the presence of a potential encounter at twice the distance given in the Dungeon Master’s Guide. Also, when a dragon makes a Spot check, it takes only half the penalty for distance: a –1 penalty per 20 feet of distance rather than the standard –1 per 10 feet of distance.
In dim light, a dragon sees four times as well as a human. The dragon’s low-light vision is exactly like that of other creatures with the low-light ability, except that the dragon sees four times as far when using artificial illumination. For example, from a dragon’s point of view a light spell produces bright illumination in a 40-foot radius around the affected object and dim light for an additional 40 feet.
In complete darkness, a dragon relies on darkvision and blindsense. Both are exactly like the standard abilities, except for the dragon’s exceptional range: 120 feet for darkvision and 60 feet for blindsense.
In addition to their superior visual apparatus, dragons commonly have ranks in the Spot skill.