How to choose camera lens focal length: 2.8 / 4 / 6 / 8 / 12mm field of view comparison

The relationship between focal length and angle of view

The shorter the focal length, the wider the viewing angle, and the "broader" the view, but the smaller the details; the longer the focal length, the narrower the viewing angle, the "further" the view, but the smaller the coverage area. The following data are based on the measured reference values of the lateral field of view of a 1/2.8″ sensor (common 2-megapixel models).

On-site estimation method

A simplified formula can be used to estimate the required focal length:

Focal length ≈ (sensor width × monitoring distance) ÷ width to be covered

For example, a 1/2.8″ sensor is about 5.4mm wide and needs to cover an 8-meter-wide entrance and exit 10 meters away: 5.4 × 10 ÷ 8 ≈ 6.8mm. You can choose 6mm or 8mm.

Fixed focus or zoom

• Fixed focus (2.8/4/6/8/12mm): low cost, large aperture, stable imaging, suitable for regular scenes with clear points.
• Manual zoom (2.8~12mm): It can be adjusted on site during installation, suitable for projects where the point position is difficult to accurately measure in advance.
• Electric zoom/dome camera: Can be adjusted remotely, suitable for key areas that require both panoramic and close-up views.

Common mistakes

• Use 2.8mm in a 20-meter-long corridor: Although you can see everything, the face at the far end has only a few dozen pixels and cannot be identified.
• Using 8mm in the elevator: the viewing angle is too narrow and can only capture a corner and cannot cover the entire car.
• Ignore sensor size:For the same focal length, a 1/1.8″ sensor has a wider viewing angle than a 1/2.8″ sensor.