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Field guide · Cabling & low-voltage

Camera Lens, Field of View and Pixel Density

Choosing a lens is two questions, not one: how much scene fits in the frame, and whether there are enough pixels on a subject to be worth anything afterwards. Covers focal length against sensor format, and the DORI pixel-density bands that decide whether footage identifies somebody or merely shows that somebody was there.

Diagram

Covering the room is not the same as identifying a face in it.

Work from

The angle from the camera's datasheet. Use this whenever you have it.

Nominal — take it from the datasheet.

84°
10 m
At 10 m
84°Field of viewHorizontal angle.
18.0 mScene widthWhat fits across the frame.
142 px/mPixel densityHorizontal pixels per metre of scene.
Which means

Recognise

Enough to say with a high degree of certainty that this is somebody already known to the viewer.

With this lens, on this camera

Each band holds out to its distance and no further. This is the figure to design against — put the door inside the identify range, not merely inside the frame.

  • Identify≥ 250 px/mto 5.7 m — not reached
  • Recognise≥ 125 px/mto 11.4 m — reached at the selected distance
  • Observe≥ 62.5 px/mto 22.7 m — reached at the selected distance
  • Detect≥ 25 px/mto 56.9 m — reached at the selected distance

Two things this arithmetic cannot tell you. It assumes the subject is square-on and evenly lit — a face at 45° or backlit against a doorway needs considerably more pixels than the band suggests. And it says nothing about frame rate: identification from a still is one problem, following somebody across a lobby is another.

Fig. 01Focal length and sensor format give the field of view; resolution and distance give the pixel density. The bands underneath are the distances each DORI level actually holds to — which is the figure to site a camera against.

Questions

Questions people actually ask

What focal length do I need for a camera?
It depends on the sensor and the distance, not on the lens alone. A 4 mm lens on a 1/2.8-inch sensor gives roughly a 65° view; the same lens on a smaller sensor is narrower. Work from the distance to the subject and the pixel density you need at that distance.
What is DORI?
Detect, Observe, Recognise, Identify — the pixel-density bands from IEC/EN 62676-4, measured in pixels per metre of scene width. Detect needs 25 px/m, Observe 62.5, Recognise 125, and Identify 250. They are the standard the industry quotes when it claims a camera can identify at a distance.
Does more megapixels mean I can identify further away?
Yes, proportionally — pixel density scales directly with horizontal resolution at a given field of view. But it also multiplies the storage and the bandwidth, and it does nothing about a subject that is backlit, at an angle, or moving too fast for the shutter.
Why does my camera cover the area but the footage is useless?
Because coverage and pixel density are different measurements. A wide lens fits the whole car park in frame while putting perhaps 20 pixels across a face — enough to say a person was present, and nothing more. Design against identify distance, not against frame coverage.
The tool for this

Camera Storage Calculator

Settled on lenses and coverage? The storage calculator turns camera count, resolution and retention into the array size you need.

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