FOCUSING

Auto Focus: 

One Shot: For stand by subject. Using "One Shot", after half clicking the shutter button the focus gets locked. We normally use our camera on One Shot mode. For eg: for clicking portraits use one shot mode. 

AI Servo: Whenever subject is moving and we want the focus to change continuously, we use AI Servo mode. Whatever comes in the center, it keeps changing the point of focus. For situations like, when subject is coming towards the camera, say you are standing on a railway track and to click the photo of a train coming towards you: Use  "AI SERVO". While doing birds photography, point of focus needs to be continuously changed  so for such cases AI Servo mode would be effective.

AI Focus: It is the hybrid of AI Servo and One Shot modes. For very slow motion of subject like moving bicycle. 

NOTES: 

- While using AI Servo mode, try moving your camera with moving subject. 

- Focus the subject and let the background go out of focus.

- You need to have enough patience to get properly focused photos.   

- For MACRO photography, use Manual Focus. It is because on AF mode camera is unable to judge what subject we want to focus.