Screenshot #1 - the zombie is obscured by my iron sight. The zombie saw me all the way from the middle of the road ahead(way behind the red house), and began running towards me.
Screenshot #2/3 - Zombie spots me across 150 meters of fence and shacks, and sprints straight towards me until termination. In the third picture, the zombie is a tiny dark mark over the fence.
Screenshot 4 - Another Zombie who easily spots me across 150-200 meters while my character is crouched.