walk me through a dragons keen senses.

[Moon-Lancer]

How does keen senses work for true dragons? as in how do you actually use this ability in the game? what are its mechanics?

is it just low light vision 4x and spot checks take a -1 for every 20 feet instead of 10?

 

[alice_of_ebony]

Keen Senses (Ex)

A dragon sees four times as well as a human in shadowy illumination and twice as well in normal light. It also has darkvision out to 120 feet.

With a sunrod, the human can see as if in bright light to 30 ft., and shadowy illumination to 60 ft. The dragon can see 2x30 = 60 ft of bright light, 4x60 = 240 ft of shadowy illumination, and don't forget that darkvision 120 ft. (or the blindsense to 60 ft).

I'm not sure where your reduced Spot check comes from though.

 

[phindar]

Well, if a dragon sees "twice as well in normal light", outside in the daylight it would make sense to give it a -1 per 20' to Spot checks, rather than the normal -1 per 10'.