Problem with lens

if you cant afford it read this,
The reason for this is that I find it easiest to track a moving subject if it's in the middle of the viewfinder. Cameras cannot know what you want to focus on, and if you start using multiple AF points, how is the camera supposed to know that you want to focus on the bird and not the tree - cameras are lacking in human thought processes! What is quite handy though is using the AI SERVO FOCUS or AI FOCUS settings so that the lens will alter it's focus on the subject over which the centre AF point is aligned as it moves towards you or further away from you. Without this, the focus will fix at the point you half depress shutter and if the subject comes closer or goes further away then it will be out of focus. you can call any expert of lences also for it if you can afford.