As I read it;
If your target is harder to see (perhaps because of partial lightning, you are trying to shoot through smoke, you got something in your eye - like a splinter, pepper punch, seven-7 etc, affected by a flashbang or within range for a flash pak, or whatever) you seem to typically get -3 dice.
If your target is much harder to see (perhaps because of dim lightning, wearing sunglasses during partial lightning, severe glare, you are caught inside the area of a smoke grenade, heavy rain, heavy snowing, close to a flash pak, target is at a far range while not using a scope, or whatever) you seem to typically get -6 dice.
If you are unable to see your target (perhaps because of total darkness, blinding glare, blindfolded, caught in a raging blizzard, failed perception against target trying to hide or invisible, target fully behind cover, eyes destroyed, holding a flash pak in front of your eyes, close to a flash pak while using low light, target is at an extreme range while not using a scope, or whatever) then your attacks typically seem to automatically fail.