CCTV Lens Calculator

How the CCTV lens calculator works

This CCTV lens calculator turns a camera’s lens focal length into the things that actually matter for planning coverage: the horizontal field of view angle, the width of scene it captures at a given distance, and the DORI distances at which it can detect, observe, recognise and identify a person. Three inputs drive it — the lens focal length in millimetres, the image sensor size (which sets the sensor’s physical width), and the camera resolution (which sets how many horizontal pixels are spread across that view).

The field-of-view formula is: FOV = 2 × arctan( sensor width ÷ (2 × focal length) ). A 2.8 mm lens on a 1/2.8″ sensor sees roughly 88° horizontally; a 12 mm lens on the same sensor narrows to about 25°. The scene coverage width at any distance is then 2 × distance × tan(FOV ÷ 2) — the further out you go, the wider the scene, but the fewer pixels land on each face.

What is DORI distance?

DORI — Detect, Observe, Recognise, Identify — is an IEC 62676-4 guideline that pegs a minimum pixel density on the target for each task: about 25 px/m to detect a person is present, 63 px/m to observe activity, 125 px/m to recognise someone you know, and 250 px/m to identify a stranger reliably. Because coverage width equals horizontal pixels ÷ pixel density, the maximum distance for each level works out to horizontal pixels × focal length ÷ (target px/m × sensor width). More resolution or a longer lens pushes every distance out; a wider lens pulls them in.

Which CCTV lens should I choose in India?

For a room, porch or lift lobby, a wide 2.8 mm lens covers the whole space at close range. For a driveway, gate approach or compound wall, step up to a 4 mm, 6 mm or longer lens so faces at the far end still land enough pixels to identify. Plan coverage, not camera count: mark every entry point, decide whether you need to detect, recognise or identify at each, and pick the lens whose Identify distance comfortably exceeds where the person will be. DORI is a rule of thumb — always leave a margin for night, rain and off-centre targets.