Free tool
CCTV Lens & Field of View Calculator
Work out what a camera actually sees: the field of view, coverage width and angle for a given lens focal length, sensor size and distance to the subject.
Choosing a lens is choosing a trade
A short focal length such as 2.8 mm gives a wide view and covers a whole room, but everything in it is small and distant faces are unusable. A long focal length such as 12 mm sees a narrow strip in useful detail. There is no setting that gives both, which is why one camera covering an entire car park is almost always the wrong answer.
Work backwards from what you need the footage to prove. If it is to show that something happened, wide is fine. If it is to identify who did it, you need the detail, and that means a longer lens, a closer camera or a second camera doing that job specifically.
Mounting height changes the picture too. High up gives wide coverage and the top of heads. At two and a half to three metres you still cover ground but faces stay recognisable, which is why entrances are usually covered from near head height rather than from the roofline.