Focal length sets the angle of view. A shorter lens gives a wider view. The same pixels are then spread over a wider scene, so each metre of the scene gets fewer of them.
That pixel density, measured in pixels per metre (px/m), decides whether you can identify a face or only detect that someone is there.
Typical 4MP camera, 1/2.8" sensor, 2560 pixels wide:
| Lens | Horizontal angle | Scene width at 10m | Pixels per metre at 10m | Max distance to identify (250 px/m) | Max distance to recognise (125 px/m) |
|---|---|---|---|---|---|
| 2.8mm | ~90° | ~19.9m | ~129 px/m | ~5.1m | ~10.3m |
| 4mm | ~70° | ~13.9m | ~184 px/m | ~7.4m | ~14.7m |
| 6mm | ~50° | ~9.3m | ~276 px/m | ~11.0m | ~22.1m |
These are theoretical figures at full native resolution, before lens distortion. The identify and recognise thresholds follow the IEC 62676-4 DORI levels. Real cameras vary: many 2.8mm datasheets quote 100° or more, because sensor size and distortion differ from model to model. The datasheet is only a starting point.
- 2.8mm: a general overview of a yard, a room or a driveway where you want to see what happened. Don't expect to identify a stranger beyond about 5m.
- 4mm: the all-rounder for entries, side paths and small shopfronts. Identification is realistic out to about 7m.
- 6mm and up: doorways, gates, counters and choke points where the face is the evidence.
Charts and calculators give you degrees and metres. They don't show you what the camera will see from that eave, at that height, looking at that gate. Most installers settle it the old way: mount it, check the NVR, move it again.
FOV Pro was built by an installer with 25 years on CCTV jobs to fix that. Stand where the camera is going, pick the camera from more than 1,000 models, and see what it will see through your phone before you drill. Compare 2.8mm and 4mm on the spot, or use Find mode to work backwards from the view you need to the cameras that fit it. It's a design aid, modelled to about ±5%, so always confirm on commissioning.
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