If the camera is to be fitted indoors and in constant light a manual iris lens or fixed iris lens are probably appropriate.
If the camera is to be fitted externally or in
an uneven light source an auto iris lens should be used
The size of lens is very important.
The first thing you should decide is if you want to identify a person or vehicle etc, or if you just want to see if something is there. If you are trying to identify and look at a wide area use more cameras.
Varifocal lenses are a little bit more expensive but they do allow you to narrow or widen the field of view.
Remember the type of lens is just as important as the camera.
Step 4 help on multi camera systems
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