Pixel Density
Will this camera actually do the job at that distance? Pixels per foot and per metre, classified against the standard detect / observe / recognise / identify tiers.
Planning estimates only. This calculator is not a substitute for engineered design or manufacturer specifications. Verify the final design against OEM documentation and the codes your authority having jurisdiction enforces.
The formula
Exactly what the calculator computes, in the order it computes it. Highlighted terms are your inputs.
// Horizontal scene width at the target plane sceneWidth = distance × sensorWidth ÷ focalLength // Horizontal angle of view HFoV = 2 × arctan( sensorWidth ÷ (2 × focalLength) ) // Pixel density on target PPM = horizontalPixels ÷ sceneWidth(metres) PPF = horizontalPixels ÷ sceneWidth(feet) // Furthest distance that still meets a given tier maxDistance = (horizontalPixels × focalLength) ÷ (sensorWidth × tierPPM)
What it assumes
Each of these is a way the number can be wrong for your job. Read them before you buy hardware.
- Thin-lens approximation. Valid when the target distance is much greater than the focal length, which holds for essentially all surveillance geometry. It breaks down for macro distances.
- The target is flat and perpendicular to the optical axis. Real scenes are oblique. A face turned away, or a target at the edge of a wide-angle frame, will resolve worse than this number suggests.
- No lens distortion or vignetting correction. Wide-angle lenses stretch the periphery; density at the frame edge is lower than the calculated average across the frame.
- Pixel density is not image quality. This calculates geometry only. Lighting, exposure, motion blur, focus, compression artefacts, WDR handling, and IR performance all degrade a usable image without changing a single pixel of density. A camera that computes to "Identify" in daylight may not recognise anyone at night.
- Sensor widths are nominal. The "type" designation is a legacy measurement, not a physical dimension, and manufacturers vary. For design work, take the active area from the sensor datasheet and enter it as Custom.
- Tier thresholds follow IEC 62676-4 and the UK Home Office framework. Some US specifications use different values — 40 PPF for identification is common. Check what your specification actually requires.
- Aspect ratio is inferred from the image resolution. If the sensor is cropped or the aspect ratio differs from the sensor native, the vertical figures will be off.
Questions people ask
- How many pixels per foot do I need to identify a face?
- The IEC 62676-4 identification tier is 250 pixels per meter, about 76 pixels per foot. Many US specifications ask for 40 PPF or more for identification, so check the number your specification actually states.
- Does a higher resolution camera always identify at a longer distance?
- Only if the field of view stays the same. Pixel density depends on horizontal resolution divided by scene width, so a wider lens on a higher resolution camera can deliver fewer pixels on target than a narrower lens on a lower resolution one.
More video surveillance tools
- Lens / Field of View Solve for focal length, angle of view, scene width, or distance given the other three. The diagram updates as you type.
- VMS Storage How much storage this camera set actually needs for the retention you promised, with headroom.
- RAID Capacity and Retention Usable capacity of a recorder or storage array by RAID level, how many drive failures it survives, and how many days of video it actually holds.
- Network Bandwidth Aggregate stream load for a camera set, split into recording and live view, with uplink sizing for the access and distribution layers.
Need it designed, installed or maintained?
We design and maintain these systems for federal facilities. Bring us the drawings.