The basic rules is:
The smaller the focal length number (eg. 3.6mm, 4mm), the wider area you can see; the bigger the focal length (12mm, 16mm), the further you can see, but with narrower viewing angle.
You should choose the camera lens depending on your environment:
- If you are going to use a camera in the office or warehouse and you want to see as much as you can, you should go for a 3.6mm lens (makes wide, somewhat distant view)
- If you want to observe limited area, like entrance, or cash registery, you should go for 12mm or 16mm lens (makes narrow view and objects are closer).