Quick answer: Scene width at a distance = distance × sensor width ÷ focal length, and pixels per foot (PPF) = the camera’s horizontal pixel count ÷ that width in feet. Compare the PPF with the DORI levels of IEC 62676-4: about 8 PPF (25 px/m) to detect, 19 PPF (63 px/m) to observe, 38 PPF (125 px/m) to recognize and 76 PPF (250 px/m) to identify a person. On a 4MP camera with a 2.8 mm lens and a common 1/2.7-inch sensor, identification holds to roughly 17 ft (5.3 m); a motorized 2.8–12 mm lens at 12 mm stretches it to about 75 ft (23 m). Take the per-SKU angle from the spec sheet; the catalog prints the lens, not the angle.
What decides the field of view: focal length and sensor size together
A lens does not have a field of view on its own. The horizontal angle is set by the focal length and the width of the sensor behind it: the same 2.8 mm lens on a larger sensor sees a wider scene. The geometry is a single triangle, which is why the whole calculator fits in two lines:
Horizontal angle = 2 × arctan (sensor width ÷ (2 × focal length))
Scene width at distance D = D × sensor width ÷ focal length
Sensor width is usually inferred from the sensor format. Approximate active widths: 1/3-inch about 4.8 mm, 1/2.7-inch about 5.4 mm, 1/1.8-inch about 7.2 mm. Where the spec sheet prints the horizontal angle, use it and skip the first line; the CASE catalog pages print the lens (2.8mm Fixed on CS-I4KS L-2.8mm, Motorized 2.8-12 mm on CS-I4KS L-2.8-12mm), so the angle for a CASE SKU comes from its spec sheet.
The angle does not tell you how much detail lands on the subject. The angle is the coverage promise; pixels per foot is the evidence promise, and the two pull in opposite directions.
DORI levels: what IEC 62676-4 asks for at the target
IEC 62676-4 defines four operational levels for a person in the scene, each as a pixel density across the target. US installers work in pixels per foot; the standard is written in pixels per meter (px/m ÷ 3.281 = PPF).
| DORI level | Pixels per meter | Pixels per foot (approx.) | What it supports |
|---|---|---|---|
| Detect | 25 px/m | 8 PPF | Something is there; a person versus a vehicle can be told apart |
| Observe | 63 px/m | 19 PPF | Clothing, direction of movement, number of people |
| Recognize | 125 px/m | 38 PPF | A known person can be recognized by someone who knows them |
| Identify | 250 px/m | 76 PPF | An unknown person can be identified from the image beyond reasonable doubt |
Two cautions. The levels assume reasonable light, exposure and a subject facing the camera; a 2.8 mm lens at night with an IR illuminator reaching 98 ft (30 m) still identifies no one at 98 ft, because the pixel density there is far below 76 PPF (see IR range explained). And plate reading is not a DORI level: it has its own density, angle and shutter requirements and is confirmed per project, never implied from a megapixel count.
Calculator: sensor, focal length, distance → scene width and pixels per foot
PPF = horizontal pixels ÷ (D (ft) × sensor width ÷ focal length)
Maximum distance for a DORI level, D (ft) = horizontal pixels × focal length ÷ (sensor width × required PPF)
Worked example: a 4MP dome at an entrance
Camera: 2560 × 1440 (the Max. Resolution printed for CS-I4KS L-2.8mm), 2.8 mm lens, 1/2.7-inch sensor assumed at 5.4 mm wide. Target: a person at the door, 20 ft (6.1 m) from the camera.
- Scene width at 20 ft = 20 × 5.4 ÷ 2.8 = 38.6 ft (11.8 m).
- PPF = 2560 ÷ 38.6 = 66 PPF (217 px/m): above Recognize, below Identify.
- Identify distance = 2560 × 2.8 ÷ (5.4 × 76) = 17.5 ft (5.3 m). Beyond that, the door camera recognizes, then observes.
- Fix: move the camera closer; raise the resolution, since 8MP at 3840 pixels wide gives 99 PPF at 20 ft and identifies to 26 ft (7.9 m); or lengthen the lens, since 4 mm identifies to 25 ft (7.6 m) on 4MP.
Every figure above depends on the assumed sensor width; replace 5.4 mm with the value from the SKU spec sheet before the number goes on a drawing. Keep the printed resolution exact: CS-I4HS L-2.8mm prints 2688×1520, not 2560×1440, a five percent PPF difference.
Fixed 2.8 mm vs 4 mm vs motorized varifocal: which lens for which distance?
The table runs the identify and recognize distances for common fixed lenses and the two ends of a motorized varifocal, with the 5.4 mm sensor assumption, so you can see the trade before the site survey.
| Lens | Horizontal angle (5.4 mm sensor) | Scene width at 20 ft (6.1 m) | Identify, 4MP (2560 px) | Identify, 8MP (3840 px) | Recognize, 8MP | Typical job |
|---|---|---|---|---|---|---|
| 2.8 mm fixed | about 88° | 38.6 ft (11.8 m) | 17 ft (5.3 m) | 26 ft (7.9 m) | 52 ft (16 m) | Entrances, small lots, rooms, anywhere the subject passes within 20 ft |
| 4 mm fixed | about 68° | 27 ft (8.2 m) | 25 ft (7.6 m) | 37 ft (11.4 m) | 75 ft (23 m) | Corridors, aisles, walkways, cash points from across a room |
| 2.8–12 mm motorized, at 12 mm | about 25° | 9 ft (2.7 m) | 75 ft (23 m) | 112 ft (34 m) | 225 ft (69 m) | Gates, fence lines, dock doors seen from a distance |
| 2.7–13.5 mm motorized, at 13.5 mm | about 23° | 8 ft (2.4 m) | 84 ft (26 m) | 126 ft (38 m) | 252 ft (77 m) | Long approaches, yard entrances, pole mounts |
A fixed 2.8 mm lens is a coverage lens: wide, forgiving on aim, evidence-grade only inside roughly 17 to 26 ft (5 to 8 m). A fixed 4 mm lens buys about 43% more identify distance for about 30% less width. A motorized varifocal is bought for distance and uncertainty: when the survey cannot fix the mounting point, when the target is 50 ft (15 m) or further, or when you want to zoom and focus from the ground after the lift has gone. The selection logic in depth is in varifocal vs fixed lens, and the angle-by-distance tables in the lens angle cheat sheet.
Mounting height eats into every figure: a camera 12 ft (3.7 m) up looking at a face 5.5 ft (1.7 m) off the ground has a slant distance longer than the floor distance, and a steep angle shortens the face. The placement rules are in security camera mounting height and angle.
Examples with CASE lenses: matching the printed lens to the job
The CASE catalog prints the lens, resolution and illuminator range; the angle and sensor format are on the spec sheet. The printed rows below are the inputs, not the answer.
- CS-I4KS L-2.8mm (dome): 4MP, 2.8mm Fixed, 2560×1440@25fps, IR 98 ft (30 m), IP66, IK10, Metal+Plastic housing. A coverage dome for lobbies, back doors and small rooms where the subject is within about 17 ft (5.3 m) for identification.
- CS-I4KS L-2.8-12mm (dome): 4MP, Motorized 2.8-12 mm, 2560×1440@30fps, IR 98 ft (30 m), IP67, IK08, Plastic housing, Smart Alarm (Human & Vehicle Detection). With the 5.4 mm sensor assumption the computed identification distance at 12 mm is about 75 ft (23 m); the IR row says the scene is lit to 98 ft (30 m), so beyond 75 ft you are observing, not identifying.
- CS-I8MS SD-2.7-13.5mm (dome): 8MP, 2.7mm – 13.5 mm, 3840×2160@20fps, IR 164 ft (50 m), White Light Distance 49 ft (15 m), IP67, IK10, Metal housing, Human & Vehicle Classification. With the 5.4 mm sensor assumption, identification is computed near 126 ft (38 m) at the long end; note the 20 fps at full resolution on a fast-moving dock.
- CS-I4US L-2.8-12mm (bullet): 4MP, Motorized 2.8-12mm, 2560×1440@30fps, IR 262 ft (80 m), IP67, Metal + Plastic housing. The long IR row is for detection and observation at the far end of a yard; computed identification stays inside about 75 ft (23 m) at 12 mm with the 5.4 mm sensor assumption.
- CS-I8TS I-2.7-13.5mm (bullet): 8MP, 2.7mm – 13.5mm, 3840×2160@30fps, IR 262 ft (80 m), IP67, Metal housing, with the extended AI list including Human/Vehicle/Non-motor Vehicle/Pet Classification. The combination of 8MP, 13.5 mm and 30 fps is the long-approach choice; white light is printed for the CS-I8US I-2.7-13.5mm variant only, so make no white-light claim for this SKU.
CASE cameras carry a 6-year warranty. Low-light and full-color behavior is stated per SKU on the specification sheet and should be confirmed for the site.
What to tell your customer
“We choose each lens from the distance at which you need to identify someone, not from how much the camera can see. Wide lenses cover more and identify less; longer lenses do the reverse. Where the distance is uncertain or long, we specify a motorized zoom lens we can adjust after installation. The pixel-density figures on our layout are calculated from the camera’s published resolution and lens, and we confirm them on the live image at commissioning.”
FAQ
How do I calculate a security camera’s field of view?
Use the sensor width and focal length: horizontal angle = 2 × arctan (sensor width ÷ (2 × focal length)), and scene width at a distance = distance × sensor width ÷ focal length. With a 5.4 mm sensor, a 2.8 mm lens gives about 88 degrees and a 38.6 ft (11.8 m) wide scene at 20 ft (6.1 m). Use the spec-sheet angle where it is printed.
How many pixels per foot do I need to identify a person?
IEC 62676-4 sets Identify at 250 pixels per meter, about 76 pixels per foot, with Recognize at 125 px/m (38 PPF), Observe at 63 px/m (19 PPF) and Detect at 25 px/m (8 PPF). Divide the camera’s horizontal pixel count by the scene width in feet at the target to get the PPF you actually have, under reasonable lighting.
Is a 2.8 mm or 4 mm lens better for security cameras?
Neither; they do different jobs. A 2.8 mm lens covers about 88 degrees and identifies a person to roughly 17 ft (5.3 m) on 4MP or 26 ft (7.9 m) on 8MP with a 5.4 mm sensor. A 4 mm lens narrows to about 68 degrees and extends those distances to 25 ft (7.6 m) and 37 ft (11.4 m). Pick by the identify distance the scene needs.
When is a motorized varifocal lens worth specifying?
When the target is 50 ft (15 m) or further, when the mounting point is not fixed at quoting time, or when the lens must be adjusted after installation without a lift. A 2.8–12 mm lens at 12 mm identifies to about 75 ft (23 m) on 4MP; CS-I4KS L-2.8-12mm and CS-I4US L-2.8-12mm print that lens, with 98 ft (30 m) and 262 ft (80 m) IR respectively; the identify distance is computed, not printed.
Does a longer IR range mean I can identify people farther away?
No. IR range is how far the illuminator lights the scene; identification depends on pixel density at the target. CS-I4US L-2.8-12mm prints 262 ft (80 m) IR, but at 12 mm the computed identify distance is about 75 ft (23 m) on its 4MP sensor. Beyond that the camera detects and observes a lit scene, which is still useful for alarms and tracking.
