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What will I see with which mm?
Field of view, frame width and pixels per metre (DORI standard) — from what distance a face, a plate or motion is visible.
How is it calculated?
The horizontal field of view is derived from the focal length, and the frame width at the given distance is: width = 2 × distance × tan(angle/2). The camera's horizontal resolution divided by the frame width gives pixel density (px/m), which is compared with the DORI levels. Example: 4 MP (2560 px), 2.8 mm, 10 m → frame ≈ 24.7 m → ≈ 104 px/m: the event is visible, but a face cannot be recognised.
Frequently asked questions
What is DORI?
DORI (IEC EN 62676-4) defines 4 levels by pixel density: Detection (25 px/m), Observation (62 px/m), Recognition (125 px/m) and Identification (250 px/m).
2.8 mm or 6 mm?
A short focal length gives a wide angle (2.8 mm ≈ 100°) — good for rooms and yards, but details are smaller. A longer one gives a narrow angle (6 mm ≈ 55°) — to see faces clearly at entrances, gates or checkouts.
Why choose a varifocal lens?
With a varifocal lens (e.g. 2.8–12 mm) the field of view can be adjusted after installation. If the distance is not exactly known or may change, it is the safest choice.
What do I need to read licence plates?
Plates need at least 250 px/m (identification) and a proper angle. For moving vehicles dedicated LPR/ANPR cameras with a fast shutter are recommended.