If you look in your manual, you will find a diagram of the focus points the camera uses. There are eleven of them.
- Your camera allows you to let the camera select the point to use, which generally is the closest thing to the camera,
- The center point only, or
- Any one of the eleven points by selecting it ahead of time.
In a studio for still life, there might be an advantage to set the AF system to focus on the point that is on an off centre subject. Personally, I have never used the selector other than to try it out when I received the camera. My brain is confused enough without trying to remember which point on my focusing screen is in use.
My personal preference is for center point AF only. Perhaps the fact that I use M series lenses that can only use the centre point may influence my decision.
Hope this helps.
The lever on the front of the camera allows you to turn off the AF system and focus yourself (MF)
Focus automatically and only release the shutter when the camera detects an in focus image (AF.S) - s for single.
Focus automatically, but take a picture whenver you press the shutter release, whether or not the camera has locked fous.