to focus properly, u must understand a bit abt how the camera achieve focus.. one of the method the camera uses to achieve focus is thru usage of contrast i.e. if the object has black and white, red and blue etc.. such contrasting colour, the camera will know that the object is there and hence hook focus there.
However if it is a smooth plain colour background, the camera sees no contrast and "feels" that there is "eternity" in that area and hence it zooms back and forth just to hunt for something of contrasting colour so that it can achieve focus.
or imagine this scenario: lets say u are in planet jupiter, whereby the people living there never wear any clothes. To find your friend amidst the huge bunch of people in a place is very difficult right? because all ppl have the same skin tone and colour and hence u can't "lock focus on your friend". But if your friend is in singapore and everyone wear clothes on the street, if your friend tells you he wear red shirt, when u look around and u saw the red contrasting colour from the background, you can "focus" on your friend liao rite?
AF illuminator works in a dark situation, such that it lights up a area, which enables it to clearly see wat objects are there, and then focus on the item u want to focus inside the illuminated area.
Think of it as u are in a dark cave hunting for dinosaur. If it is totally dark, u cant see anything and thus u cant focus on anything. If u light up a torch, u can see the items in the cave and then u can focus liao rite?
