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.
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.