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Choosing the Right CCTV Lens: Coverage vs. Identification Distance
The most common lens mistake isn't a wrong formula — it's picking a wide lens because it "covers more," then finding out during an actual incident that nobody in the footage can be identified.
Coverage and detail are opposite trade-offs
A wider lens (shorter focal length) spreads the same sensor resolution across a bigger scene — more area covered, fewer pixels per meter of that area. A narrower lens (longer focal length) does the opposite: less area, but every object in frame gets more pixels. Neither is "better" — they're suited to different jobs, and the mistake is picking one without checking what the resulting pixel density actually gives you at the distance that matters.
Run those two together and you get the number that actually matters on site: how many pixels per meter fall on a person standing at your entrance, your gate, or the far end of your parking lot. The Camera Field of View Calculator does this automatically from sensor size, focal length, resolution, and distance.
Johnson's Criteria: the industry's own yardstick
Every major manufacturer's own lens-selection tool (Axis, Bosch, Hikvision, Dahua) works off the same three thresholds, because they come from established resolution-target research, not marketing:
| Classification | Pixel density | What you can actually do with it |
|---|---|---|
| Identification | ≥ 125 px/m | Confirm exactly who a person is, or read a plate |
| Recognition | ≥ 63 px/m | Recognize someone you already know, or a vehicle type/color |
| Detection | ≥ 25 px/m | Notice that a person or vehicle is present at all |
Below 25 px/m, the footage is only good for general scene awareness — motion happened, but you can't say who or what caused it. This is the gap that catches technicians most often: a camera spec sheet's megapixel count says nothing about which of these three tiers you're actually getting at your specific install distance.
The mistake: sizing the lens for the room, not the job
A common install pattern: pick a 2.8mm lens because it "gets the whole room in frame," mount it, and move on. That lens might genuinely cover the whole space — and still leave every face in it below detection grade the moment someone is more than a few meters from the camera. The lens did its job of showing the area; it never had enough resolution per meter to show who was in it.
The fix isn't a longer lens everywhere — a narrow lens at a doorway loses the wide coverage you need to see someone approaching. It's matching the lens to the job at each specific mounting point: wide coverage where general awareness is the goal (a parking lot overview), longer focal length where identification is the goal (an entrance, a till, a gate).
Sensor size matters as much as focal length
Two cameras with the same focal length but different sensor sizes have different fields of view — a larger sensor behind the same lens sees a wider angle, not a narrower one. When comparing lens specs across camera models, focal length alone is not comparable unless the sensor size is the same; this is why the field of view calculator asks for both.
Putting it together
Identify the distance where each camera actually needs to deliver identification, recognition, or just detection. Run sensor size, focal length, resolution, and that distance through the Camera Field of View Calculator to see which tier you land in — and, if it's short, whether a longer lens or a higher-resolution sensor closes the gap more cheaply. Treat the result as a best case: lens distortion, compression, and low light all eat into it in the field.
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