A CCTV camera lens size is expressed in millimeters (MM) and determines the field of view & zoom.

Typical FIXED camera lenses are 2.8mm,  3.6 or 4mm,  6mm,  8mm or 12mm. A larger sized lens has a narrower & “zoomed in” picture/view.

Examples of typical lens size vs viewing angles are as follows:
2.8mm = ~ 84-115°
3.6/4mm = ~ 67-90°
6.0mm = ~ 44-55°
12mm = ~ 26-33°
* Note angles will vary depending on the manufacturer *

In some cases we recommend a “varifocal lens” so that you can adjust the lens to the exact view you need.

Hopefully the following pictures will help explain the info above: