A higher attenuation factor in the detection formula enables NPCs to see you realistically in very low light levels when very near, without making detection over-powered at further distances (same applies to hearing).However, in total darkness (with a mod like ELFX Hardcore) NPCs are just as blind as in vanilla.

Even equipping a spell that lights up your hand will get you spotted. They can now see better in dimly lit environments, and will quickly spot you if you are carrying a light source. Enemies won't become deaf and blind as you increase your sneak level and perks.