Lens Selector Tool

How focal length, sensor and DORI interact

Focal length is the distance, in millimetres, between the optical centre of the lens and the imaging sensor when the lens is focused at infinity. Shorter focal lengths capture wider angles; longer focal lengths capture narrower angles with more apparent magnification. The same lens behaves differently on different sensor sizes — a 4 mm lens on a 1/3" sensor gives 65° HFOV, the same 4 mm lens on a 2/3" sensor gives 95° HFOV. Lens selection without sensor selection is meaningless.

For installer practice, the four EN 62676-4 DORI thresholds map cleanly to focal-length recommendations once distance and sensor are fixed. On a 1/2.8" 4 MP camera (the modal CCTV configuration in 2026), the rules of thumb are: 2.8 mm for Detect-grade coverage of areas up to 5 m, 4 mm for Observe-grade coverage of 5–10 m, 6 mm for Recognize-grade coverage of 10–15 m, 8 mm for Recognize at 15–20 m or Identify at 8–10 m, 12 mm for Identify at 12–18 m, and 16–25 mm for Identify beyond 20 m. The calculator above applies a simplified version of this mapping to the line-of-sight distance you input, accounting for mounting height via the slant range.

Fixed lenses commit you to one focal length; varifocal lenses (e.g. 2.8–12 mm) let you tune on-site after the camera is mounted. Fixed lenses are typically 30–50% cheaper, slightly sharper at full aperture, and have fewer moving parts to fail. Varifocal lenses are the right choice when (a) the install distance is uncertain, (b) the customer might rearrange furniture or shelving, or (c) you are deploying a single SKU across many sites and want to standardise inventory. Motorised varifocal — sometimes called "auto-focus" or "remote zoom" — adds the ability to adjust from the VMS without revisiting the site, which pays for itself after one site visit avoided.

Fish-eye lenses (1.0–1.8 mm, often M12-mount) achieve 180–360° hemispherical coverage by deliberately distorting the image. The pixel density at the edges is dramatically lower than at the centre, so the effective DORI range of a fish-eye is much shorter than its angular coverage suggests. Use fish-eye for situational awareness — knowing that someone is somewhere in the room — and pair it with a separate telephoto camera for any identification task. Standard rectilinear lenses (2.8 mm and longer on conventional sensors) preserve straight lines and uniform pixel density, which is what every DORI calculation assumes.

Lens quality matters most under stress. The standard MTF (modulation transfer function) curve plotted in lens datasheets shows how much contrast the lens preserves at increasing spatial frequencies — higher MTF at high frequencies means sharper detail. Glass elements outperform plastic on MTF, thermal stability, and long-term clarity, but they cost three to five times more. For 2.8–4 mm wide-angle lenses where pixel density is already low, a budget plastic lens is fine. For 12 mm and longer where every line of MTF translates into evidentiary range, premium glass with low-dispersion (LD) elements pays back quickly.

F-stop — written f/1.6, f/2.0, etc. — is the ratio of focal length to entrance-pupil diameter and determines how much light reaches the sensor. Lower f-numbers gather more light. f/1.4 is twice as bright as f/2.0, four times as bright as f/2.8. For low-light installs (parking lots at night, indoor warehouses with poor lighting), every stop matters: an f/1.6 lens will give a usable image where an f/2.4 lens drops below the camera's minimum lux rating. The trade-off is depth-of-field and edge sharpness — wider apertures focus a narrower distance band and exhibit more chromatic aberration. For long-range identification at 20 m+ in good light, f/2.0–f/2.4 is the sweet spot. For low-light dome installs at 5–10 m, prioritise f/1.4–f/1.6.

How to use this lens selector

1. Set the mounting height. Use the actual installed height of the camera above the target plane. For ceiling-mounted indoor cameras the target plane is typically the floor; for pole-mounted outdoor cameras it is usually 1.5 m above ground (head-height). 3 m is the modal indoor mounting height, 4–6 m is typical for outdoor pole mounts.
2. Set the target distance. This is the horizontal distance from directly under the camera to the target. The calculator combines mounting height and horizontal distance into a slant range, which is what the lens actually has to resolve.
3. Read the recommended lens. The green panel shows the focal length that hits a balanced pixel-density target on a typical 4 MP 1/2.8" camera. Use it as a starting point for tender drafts and bid responses.
4. Compare against the full chart. The lens comparison table shows every common focal length with its expected coverage range and FOV. Use it to evaluate alternatives — for example, if your tender requires Identify-grade coverage at the same distance, step up two slots from the recommended lens.

Worked example: retail entrance facial-ID

A high-street retailer has a 1.8 m wide automatic-door entrance and wants every customer's face captured at the EN 62676-4 Identify threshold (250 PPM) for loss-prevention review. The chosen mount is the existing dropped ceiling 3 m above the floor, with the camera positioned 4 m horizontally inside the door so customers walk towards it as they enter.

The slant range from the camera to a face plane at 1.6 m height — assuming the face is 1.4 m below the camera — is √(4² + 1.4²) = 4.24 m. Plug those values into the lens selector with mounting height 3 m and target distance 4 m and the recommended lens is 2.8 mm. But that recommendation is calibrated to balanced general-purpose coverage; for Identify-grade pixel density on a face we need to verify with explicit DORI math.

On a 4 MP 1/2.8" sensor (2560 horizontal pixels, 5.4 mm sensor width), a 2.8 mm lens gives HFOV ≈ 88°, scene width at 4.24 m ≈ 8.2 m, and pixel density of about 312 PPM — comfortably above the 250 PPM Identify floor. The horizontal coverage of 8.2 m is far wider than the 1.8 m doorway, so a single camera covers the entrance with margin to capture customers approaching from either side. A 4 mm lens would give 478 PPM and 5.7 m coverage — also workable, with extra evidentiary margin in exchange for slightly tighter horizontal capture.

The integrator picks a 2.8–12 mm motorised varifocal as the SKU for the bid because (a) the chain has 80 stores with varying door widths and ceiling heights, and (b) any future store layout change can be re-tuned remotely from the VMS without dispatching a technician. Total cost premium over a fixed 2.8 mm SKU is about 35%, but the avoided-truck-roll savings break even at one site visit prevented per camera over its 5-year service life.

Common lens selection mistakes

• Picking a lens for the maximum distance instead of the working distance. A 25 mm lens covers 50 m beautifully but is useless at 5 m — anything closer is out of focus, and the FOV is too narrow to catch the subject. Always choose the lens for the typical working distance, not the worst case. If the working range varies, use varifocal.
• Confusing optical zoom with digital zoom. A 12× optical zoom genuinely increases pixel density on the target. A 12× digital zoom just upscales fewer pixels in software — it cannot create detail that the lens did not capture. Identification requirements always need optical reach.
• Ignoring f-stop in low-light installs. A 4 mm f/2.4 lens at 5 lux is about half as bright as a 4 mm f/1.6 — that often means the difference between a usable colour image and a noisy black-and-white IR-only frame. Always check minimum-illumination specs in the lens-camera combination, not the camera alone.
• Mismatching lens mount to sensor format. A lens designed for a 1/3" sensor will vignette badly when used with a 1/2" sensor. Always match the lens image-circle specification to the sensor size or larger. M12 mounts dominate up to 1/2"; CS-mount dominates 1/2" and larger.
• Over-specifying glass quality on wide-angle lenses. A 2.8 mm wide-angle lens already spreads pixels thinly — premium glass barely improves usable resolution at distance. Save the budget for the longer-range lenses where MTF actually translates into evidentiary range.
• Forgetting tilt foreshortening on long lenses. A 25 mm lens aimed steeply down compresses the depth-of-field dramatically. Persons at the near edge of the frame look distorted; persons at the far edge look the right shape but tiny. Long lenses want shallow tilt angles; deep tilts want shorter focal lengths.

Standards and compliance references

• EN 62676-4:2015 — Application guidelines for video surveillance. The lens recommendations above are calibrated to the standard's 25 / 63 / 125 / 250 PPM thresholds. EN 62676-4 calculator →
• IEC 62676-4:2025 (OODPCVS) — The 2025 update that introduces corridor-mode pixel density and AI-analytics sub-tiers; relevant when picking lenses for hallway and machine-vision deployments.
• NATO STANAG 4347 / Johnson Criteria — Cycles-on-target metric for thermal sensors. Drives lens selection for long-range thermal imaging where DORI does not apply. Johnson Criteria calculator →
• NDAA Section 889 — US procurement restriction on listed manufacturers; applies to camera-and-lens assemblies sold as a unit. NDAA compliance reference →
• ISO 12233 — Resolution and spatial-frequency response measurement methodology. The basis for MTF measurements quoted in lens datasheets.

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