Fit check
CCTV Lens and Coverage Calculator
Fix the focal length before you fit anything. Enter lens (mm), sensor, resolution, mounting height (m) and distance to the subject (m); the result gives horizontal and vertical angle (°), scene width (m) and pixel density (px/m), checked against the four DORI levels, with a plan view of the coverage.
Field of view
85°
horizontal angle
Vertical angle
54°
top to bottom of the picture (16:9 sensor)
Scene width at 10m
18.2 m
what the frame covers
Detail level
135 px/m
Recognise: a familiar person, or a plate read (125 px/m)
| DORI level | Needs | Furthest distance |
|---|---|---|
| Identify | ≥ 250 px/m | 4.8 m |
| Recognise | ≥ 125 px/m | 10.9 m |
| Observe | ≥ 62 px/m | 22.5 m |
| Detect | ≥ 25 px/m | 56.3 m |
Coverage from above
Plan view. The wedge is the horizontal field of view; the arcs mark the ground distance where each DORI level ends, as far as the diagram reaches.
All five lenses compared
2.8, 4, 6, 8 and 12 mm side by side, for the sensor and resolution above. Drawn from above: each wedge is the horizontal angle, out to the distance where a face can still be recognised (125 px/m).
| Lens | Angle | Identify (250 px/m) | Recognise a face (125 px/m) | Observe (62 px/m) |
|---|---|---|---|---|
| 2.8 mm | 85° | 4.8 m | 10.9 m | 22.5 m |
| 4 mm | 65° | 7.5 m | 15.8 m | 32.3 m |
| 6 mm | 46° | 11.7 m | 24 m | 48.6 m |
| 8 mm | 35° | 15.8 m | 32 m | 64.8 m |
| 12 mm | 24° | 24 m | 48.2 m | 97.3 m |
Light, angle and motion cost detail, so treat the distances as the best case. Pick the lens whose Recognise distance is longer than the furthest point you must cover.
Formulas used
- Horizontal angle (°) = 2 × atan(sensor width ÷ (2 × focal length))
- Scene width (m) = distance (m) × sensor width (mm) ÷ focal length (mm)
- Pixel density (px/m) = horizontal pixels ÷ scene width (m)
- Reach (m) = horizontal pixels × focal length ÷ (DORI px/m × sensor width)
Line-of-sight distance (m) = √(mounting height² + ground distance²); px/m above uses the scene width at this line-of-sight distance, so it is a little wider than the width across the floor shown here
Varifocal lens: run the calculator at each end of its range (for example 2.8 mm, then 12 mm). The wide end sets the angle; the long end sets the reach.
Where to fit it
Once the lens is set, the mounting decides what it sees: a lower mount gives a steeper view of faces, a higher one a wider view of the ground. Fittings listed here:
Reading the figures
Focal length versus detail: 2.8mm gives a wide scene at low px/m; 12mm gives a narrow scene with high px/m at distance. DORI thresholds per IEC 62676-4, in px/m: 25 Detect (movement), 62 Observe (the event), 125 Recognise (a familiar face; a number plate), 250 Identify (an unknown person). They are common guidance, not a guarantee: light, angle and motion all cost detail.
Mounting height changes the distance that counts. The lens looks down the straight line from the mount to the subject, so a higher mount makes that line longer than the walk along the ground, and px/m falls with it. Typical layout: a short focal length at the entrance for context; on the approach, a longer focal length or more pixels, for identification. A varifocal lens can do both, adjusted on site.