IOTKAMLens calibration
How we focus a body worn camera — and why it decides whether a face is usable
A body worn camera is not focused on whatever happens to be in front of it at the moment the lens is turned. It is focused at a single computed distance, chosen so that a subject at arm’s length and a subject at the far end of a street both land inside the depth of field. This page sets out how that distance is calculated, how the station is set up, and how you can check the result on any camera with a tape measure.
The short answer
A body worn camera is focused at its hyperfocal distance — for a typical wide-angle lens around 0.7 to 0.8 m. At that setting the depth of field runs from roughly half that distance to infinity, so 0.4 m and 30 m are both acceptably sharp. Focusing closer trades away far-field sharpness you actually need, in exchange for near-field sharpness you largely do not.
Getting that distance right is arithmetic, not art. Getting the field of view right is where most specifications go wrong, because a wide-angle lens does not follow the lens formula printed in textbooks.
The inputs
Three numbers decide the whole setup
Everything downstream — sharpness, field of view, how many pixels a face receives — comes out of focal length, aperture and the physical size of one photosite.
01
Focal length
For a fixed-focus body worn camera this is deliberately short — usually 2.8 to 3.2 mm. Short focal length buys the wide field of view the job requires, and as a side effect makes the depth of field very deep even at a wide aperture.
02
Aperture
f/2 to f/2.4 is typical: fast enough for indoor and dusk recording, and because the lens is wide, the loss of depth of field is small. A faster lens does not meaningfully help a camera that is already focused past its hyperfocal distance.
03
Pixel pitch
Sensor width divided by horizontal pixel count. A 1/2.9″ sensor 5.376 mm wide with 1920 photosites gives a 2.8 µm pitch. This single number sets the circle of confusion, and therefore the hyperfocal distance.
From pixel pitch to hyperfocal distance
We allow a blur circle of two photosites — the point at which defocus is no longer distinguishable from the sensor’s own sampling limit. From there the chain is short:
c = k × pitch → 2 × 2.80 = 5.60 µm
H = f² ÷ (N × c) + f → 3.15² ÷ (2.3 × 0.0056) + 3.15 = 774 mm
So for this lens the hyperfocal distance is 0.77 m. Focused there, the near limit of acceptable sharpness sits at 0.39 m and the far limit at infinity.
Depth of field
Why we focus far away instead of on the subject
It feels wrong to point a camera at a chart two metres away and then focus it at 0.77 m. Here is what the two choices actually buy.
There is a second reason, specific to this product category. Evidence footage is mostly examined for identification — a face, a plate, a reflection in a window. Those subjects are usually well away from the officer, not on the officer’s chest. Optimising for the near field optimises for the part of the frame nobody argues about.
Field of view
The wide-angle trap: why the textbook formula understates the frame by 25%
This is the part that separates a credible field-of-view figure from a marketing one. It also decides whether the test chart you buy is the right size.
The familiar relationship between image height and angle is r = f · tan θ. It describes a rectilinear projection, and it is exact — for a lens with no distortion. Point a rectilinear lens at a tall building and straight lines stay straight, but the corners stretch, because the projection has to map an ever-larger angle onto an ever-larger radius.
A body worn camera lens cannot do that with a 100° field. To keep the image roughly proportional to angle instead of to tangent, wide lenses follow one of three other mappings. All four are in use, and they disagree sharply at the edges of the frame:
| Projection | Mapping | Horizontal FoV | Vertical FoV | Frame width at 1.2 m |
|---|---|---|---|---|
| Rectilinear | r = f·tanθ | 81.0° | 51.3° | 2.05 m |
| Equidistant | r = f·θ | 97.8° | 55.0° | 2.75 m |
| Equisolid angle | r = 2f·sin(θ/2) | 101.0° | 55.5° | 2.91 m |
| Orthographic | r = f·sinθ | 117.2° | 57.4° | 3.93 m |
Same 3.15 mm lens, same 1/2.9″ sensor, same 1920 × 1080 pixels. Only the projection model differs.
In practice, lenses with roughly 20% barrel distortion — which is what most body worn camera lenses carry — sit between the equidistant and equisolid mappings. Neither is “correct” in the abstract; the right one is whichever the lens actually is. That is not something a datasheet will tell you reliably, which is why we measure it. The last section of this page shows how you can measure it too.
Resolution
How far away can it see a face?
The honest answer is a pixel count, not a distance. Given the geometry, the count at each distance follows directly.
| Distance | Face width, pixels | What that supports |
|---|---|---|
| 0.5 m | 335 px | Identification, with detail |
| 1.0 m | 168 px | Identification |
| 2.0 m | 84 px | Recognition |
| 3.0 m | 56 px | Description only |
| 5.0 m | 34 px | Silhouette |
Assumes a 150 mm face width, a 3.15 mm lens, equidistant projection and a 1920-pixel horizontal count. Face turned away from the camera, low light, motion and video compression all reduce the effective figure.
This is the calculation we would rather have a customer run before a project than after a pilot. It is also why we publish it: a supplier who quotes a recognition distance without stating the pixel count, the lens and the sensor size has not told you anything you can verify.
The station
How the calibration station is set up
Once the lens is characterised, the station is solved backwards from a target: how much of the frame should the test chart fill?
A chart fills too little of the frame and the operator is judging focus from too few pixels; fills too much and the chart runs into the extreme edges of a wide-angle image, where distortion is at its worst and straight edges are no longer straight. The working band we use is 35% to 60% of frame height, with anything above 78% treated as out of frame.
For a given chart, the distance that produces a target fill is found by inverting the projection, which has no closed form for all four models — so it is solved numerically. With a 420 mm chart height and the lens above, 60% fill lands at 0.71 m. But the hyperfocal distance requires 0.77 m. Taking the larger of the two gives 0.77 m, where the chart fills 55% of the frame — inside the band, and past the hyperfocal distance.
Calculator
Run the numbers for your own configuration
Change any input and the results update. The default is a typical 1080p wide-angle body worn camera; the page works without scripting too, and carries the default results as static values.
Inputs
Projection model
Not sure which one your lens is? Measure it — see the tape-measure method below.
Runs entirely in your browser. No libraries, no network requests, nothing sent anywhere. You can save this page and use it offline.
Recommended focus distance
The hyperfocal distance for this lens. Focused here, 0.39 m to infinity stays within the depth of field.
Chart fill against distance — green band is the 35–60% working range
Frame size and chart fill across distance
Narrow screen — swipe the table sideways to see every column.
| Distance | Frame width | Frame height | Chart fill | Chart pixels | Status |
|---|---|---|---|---|---|
| 0.5 m | 1.15 m | 0.52 m | 82.8 % | 895 px | Out of frame |
| 0.8 m | 1.83 m | 0.83 m | 53.5 % | 578 px | Usable |
| 1.0 m | 2.29 m | 1.04 m | 43.1 % | 466 px | Usable |
| 1.2 m | 2.75 m | 1.25 m | 36.1 % | 390 px | Usable |
| 1.5 m | 3.44 m | 1.56 m | 29.0 % | 313 px | Too small |
| 2.0 m | 4.58 m | 2.08 m | 21.8 % | 235 px | Too small |
| 3.0 m | 6.88 m | 3.12 m | 14.6 % | 157 px | Too small |
Face width in pixels across distance
Narrow screen — swipe the table sideways to see every column.
| Distance | Face width | What that supports |
|---|---|---|
| 0.5 m | 335 px | Identification, with detail |
| 1.0 m | 168 px | Identification |
| 2.0 m | 84 px | Recognition |
| 3.0 m | 56 px | Description only |
| 5.0 m | 34 px | Silhouette |
Verification
How to check the field of view of a camera you already have
You do not have to take our word for any of the figures above, and you should not take another supplier’s word either. A tape measure settles it in about five minutes.
- Set the camera up at a known distance. Clamp it at the position it would occupy on a wearer, and measure the distance from the lens front to a blank wall. One metre is convenient; write down the exact figure.
- Mark the horizontal edges. Put a strip of tape on the wall at each side of the live image, exactly where the picture ends. Measure the span between the two strips.
- Repeat vertically. The top and bottom edges are harder to judge by eye — if a chart happens to fill the frame vertically, measure the chart instead. Otherwise use a straight edge held up to the image.
- Feed the two measurements back into the calculator above. Compare the resulting field of view with what the datasheet claims. A gap of more than a few percent means the datasheet is using a different projection, or a different definition of the frame.
Questions
Common questions
What is the hyperfocal distance of a body worn camera?
For a typical wide-angle body worn camera — around 3 mm focal length at f/2.3, with a 2 MP sensor of 2.8 µm pixel pitch — it works out to roughly 0.77 m. Focused there, the depth of field runs from about 0.39 m to infinity, which is why a subject at 1 m and a subject at 30 m can both be acceptably sharp.
How far away can a body worn camera identify a face?
It is a geometry question, not a marketing one. With a 3.15 mm lens and a 1920-pixel-wide sensor, a 150 mm face occupies about 168 pixels at 1 m, 84 at 2 m, 56 at 3 m and 34 at 5 m. Identification is generally treated as reliable from about 120 pixels upward, recognition from 80, and description only below 60. Light, motion and compression all reduce the effective figure.
Why does a wide-angle lens need a different field-of-view formula?
The familiar formula assumes a rectilinear projection, which only holds for narrow lenses. A body worn camera lens with roughly 20% barrel distortion follows an equidistant or equisolid mapping instead. Applying the rectilinear formula to it understates the horizontal field by about 25% — 2.05 m instead of 2.75 m at 1.2 m in the example above.
Does focusing past the hyperfocal distance hurt close-up image quality?
Only slightly, and only below the near limit — 0.39 m for the example lens. In exchange the entire far field is sharp. For evidence work that trade is almost always the right one, which is why fixed-focus body worn cameras are set this way rather than focused on the subject in front of them.
Can I use this calculator for a different lens or sensor?
Yes — enter the focal length, aperture, sensor dimensions and pixel count directly, and pick the projection model you measured. The formulas are the standard ones and are not specific to our products. If you would rather we ran the numbers for a specific configuration, send us the details and we will.
Next step
Have a lens specification you want checked?
Send us the sensor, focal length and the field-of-view figures you have been quoted. We will tell you which projection those figures are consistent with — and whether they can support the recognition distance your project needs.