How it works
The formula, without the black box.
scene width = distance × sensor width ÷ focal length; px/m = horizontal pixels ÷ scene width
The rectilinear thin-lens approximation also gives horizontal angle: 2 × arctan(sensor width ÷ 2 × focal length). Real lenses and camera processing can crop or distort the result.
Inputs that deserve a second look
- Active sensor width
- Physical horizontal imaging width from reliable device/lens data.
- Focal length
- Actual focal length in millimetres.
- Distance
- Distance from camera to the plane where scene width is evaluated.
- Horizontal pixels
- Pixels across the configured stream after any crop or aspect-ratio change.
Common planning mistakes
- Entering 1/2.8 as if it were a 9.07 mm physical width.
- Treating centre pixel density as uniform across a tilted scene.
- Assuming a DORI threshold guarantees an operational result.
- Ignoring focus, shutter, light, compression, target angle, occlusion, and lens distortion.
Questions installers ask
What do DORI numbers mean?
They are density-based planning categories for detection, observation, recognition, and identification. They are not guarantees that a real scene or target will be usable.
Why does scene width grow linearly with distance?
For a fixed rectilinear lens and target plane, similar-triangle geometry makes width proportional to distance.
Can I use diagonal sensor size?
Not in this horizontal formula. Use active horizontal sensor width and horizontal output pixels.
Sources and boundaries
These links support the method or terminology. They do not turn a planning estimate into a site guarantee.
Axis pixel density based on IEC 62676-4 ↗
Primary vendor white paper explaining density categories and their limitations.