The field of view or image displayed
on your TV or monitor varies depending on the size of your cameras image
sensor e.g.(1/4", 1/3", 1/2") and the lens size used e.g.(4mm, 6mm
etc.). This calculator can help give you a better idea of the correct
lens size you will need to obtain your desired field of view.
The larger the lens the more detail you can
see at a particular distance but your field of view will be smaller. It
is similar to looking through stronger and stronger binoculars. The
3.6mm lens is very popular since it provides good overall coverage and a
wide field of view, however if your goal is to get facial details of
someone at a distance greater than 20 to 25 feet away you would need a
larger lens in the camera, possibly a 6mm or 8mm. So you need to
decide what your viewing goal is and whether one camera will be adequate
or if you need 2 or more.