Field of View and Focal Length: The Complete CCTV Lens Guide
Field of view (FOV) is the single most important optical specification for a CCTV camera — it determines the width of the scene at any distance, and therefore every other coverage metric (pixel density, DORI zone boundaries, ANPR plate-read range). This guide derives HFOV and VFOV from first principles using sensor width and focal length, then shows how to solve the inverse problem — "what lens do I need for a given coverage width?"
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1. The HFOV equation
A camera's horizontal field of view is a function of two physical quantities: the width of its image sensor and the focal length of its lens.
where sensor_width is the active width of the image sensor (in mm) and f is the focal length (in mm). The vertical field of view follows from the aspect ratio:
2. Worked example
A 1/2.8" sensor has a width of approximately 5.57 mm. Fit a 4 mm lens:
At 10 m, this camera sees a horizontal scene width of W = 2 · 10 · tan(34.86°) = 13.9 m. With a 1920 px sensor the pixel density is 1920 / 13.9 = 138 px/m, solidly within the Recognition tier.
Switch to a 2.8 mm lens on the same sensor: HFOV = 2 · arctan(5.57 / 5.6) = 89.5°. At 10 m the scene width jumps to 19.9 m, density drops to 96 px/m — still Observation, but no longer Recognition.
3. Sensor format sizes
| Format | Width (mm) | Height (mm) | Common in |
|---|---|---|---|
| 1/4" | 3.70 | 2.78 | Very old analogue / specialty |
| 1/3" | 4.80 | 3.60 | Older HD cameras |
| 1/2.8" | 5.57 | 3.13 | Standard 1080p–5 MP |
| 1/1.8" | 7.18 | 4.04 | Higher-end 4K–8K |
4. Solving the inverse problem
Given a target coverage width W at distance d, the required HFOV is:
Then, given sensor width, solve for focal length:
Example: you need a 20 m wide coverage at 15 m. HFOV = 2 · arctan(20 / 30) = 67.4°. On a 1/2.8" sensor (5.57 mm): f = 5.57 / (2 · tan(33.7°)) = 4.16 mm. The closest standard lens is 4 mm.
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5. Key takeaways for layout engineering
- Lower focal length = wider view, lower pixel density. A 2.8 mm lens covers a room but may not identify faces at the far end.
- Higher focal length = narrower view, higher pixel density. A 12 mm lens reads a number plate but misses the scene around it.
- Sensor size is not negotiable. Two cameras with the same focal length but different sensor formats produce different HFOVs. Always verify the sensor width.
- Design backwards from the DORI requirement. Establish which tier you need at the farthest point, calculate required HFOV, then select a focal length that delivers it.