What on earth does Focus, press the Left Alt key do apart from give the screen a yellowish tinge?
It gives you enemy and "special item" locations that are <15m away in the direction you are looking, so it utterly useless outdoors but its fantastic when you're trying to creep around indoors, doing dozens of take-downs to maintain your stealth rather than risking a silenced pistol shot. I started off thinking was useless until I replayed the end of Mission 2 (trying to get the kill challenge done) and was using take-downs rather than shooting and it genuinely changed how I played the game once I figured out how to use it and I'd say it was easily my 2nd most important "skill" in-game now.