Perceiving someone or something is accomplished with a perception or find check. Environmental conditions like lighting, magical invisibility or concealment like fog can modify the check. Effects that completely impede vision of the target like full darkness or full invisibility will add +10 to the difficulty of the check unless the character making the check has some means to overcome the condition. Night time is not full darkness and adds +3 to the difficulty of observing the target. Concealment like fog will add a variable modifier as determined by either the spell/miracle description, or by the GM for natural fog. The modifier should vary from 1 for very light fog, to 10 for fog so thick you can't see your hand in front of your face.
Targeting a character with any effect that completely impedes vision, requires a roll to determine if the target can be located. For effects that partially impede vision, The target can be automatically located, unless the target is stealthing in which case a location roll opposed by the stealth character, modified by the observation difficulty is required. Regardless of whether the location roll is successful or not, a targeting difficulty modifier is still applied.
Targeting an un-located character incurs a +10 difficulty. Targeting a located character in an environmental effect that completely impedes vision incurs a +5 difficulty modifier. Targeting a character in a condition that partially impedes vision will incur a variable difficulty modifier as determined by the spell/miracle description, or the GM for natural effects. The modifier should vary from 1 to 5.
Creatures that rely completely on alternate senses are completely un-effected by conditions that impede vision. Creatures that have enhanced senses in addition to vision, may have the difficulty modifiers reduced as determined by the GM.