Calculate field of view, coverage width and pixels per metre. Verify your lens meets detection, recognition or identification requirements.
For facial identification you need at least 250 px/m at the target distance. With a 1080p (1920 px wide) camera on a 1/3" sensor, a 6 mm lens gives roughly 2.7 m coverage width and 711 px/m — more than enough. A 4 mm lens gives 4 m coverage but only 480 px/m at 6 m, which is still adequate for identification.
Detection (25 px/m) means a person is visible. Observation (63 px/m) allows general activity monitoring. Recognition (125 px/m) allows you to identify a known person. Identification (250 px/m) provides enough detail for forensic purposes. These benchmarks come from IEC 62676-4.
Coverage width = 2 × distance × tan(FOV/2). FOV is calculated from the sensor size and focal length: FOV = 2 × arctan(sensor width / (2 × focal length)). This calculator does this automatically for the common sensor sizes.