Focal length and field of view are the two ends of the single most misunderstood number in a camera spec: the millimeter figure on the lens. Focal length — 2.8mm, 4mm, 6mm, 12mm — sets how wide an angle the camera sees and, inversely, how far away it can still identify a face or a plate. Get it right and every camera frames exactly the scene it should; get it wrong and you own either a sweeping view that cannot identify anyone or a sharp close-up that misses half the room. This guide translates the millimeters into real angles and real distances, with a lens table you can apply scene by scene.
Key takeaways
- Low mm (2.8) = wide angle, short identification range; high mm (12) = narrow angle, long reach.
- Wide and far are a trade-off — you cannot have both from one fixed lens.
- The DORI standard turns "identify at X meters" into a real pixel requirement, not a guess.
- 2.8mm suits small rooms and entrances; 4mm most shops and yards; 6mm+ long approaches and gates.
- When the ideal focal length is unclear, a varifocal lens lets you dial it in on the wall.
What focal length really means
Focal length, measured in millimeters, is simply how zoomed-in the lens is. A short focal length like 2.8mm is a wide-angle: it takes in a broad scene — a whole room, a wide storefront — but spreads its pixels thinly, so distant faces blur. A long focal length like 12mm is telephoto: it zooms into a narrow slice and reads detail far away, but sees only that slice. Everything between trades width for reach along a smooth scale. The mistake to avoid is wanting both at once from one fixed lens — physics does not allow it, and the wide-angle camera that "covers everything" is exactly the one that identifies no one.
The DORI standard: from guess to number
Professionals size lenses with the international DORI standard, which defines four levels by pixel density on the target. Detect (you can tell someone is there) needs about 25 pixels per meter of scene; Observe (see what they are doing) about 63; Recognize (a known person) about 125; Identify (prove who a stranger is) about 250. The higher the level you need, the more the lens must zoom the scene onto the sensor at that distance — which is why "identify at the gate" demands a longer focal length than "detect movement in the yard". DORI turns the vague ask into a concrete lens choice: decide the level, decide the distance, pick the millimeters.
The lens table by scene
| Focal length | Angle | Face identification (2MP, approx.) | Typical scene |
|---|---|---|---|
| 2.8mm | Very wide (~100°) | Up to ~3 m | Small rooms, entrances |
| 4mm | Wide (~80°) | Up to ~5 m | Shops, gardens, yards |
| 6mm | Medium (~55°) | Up to ~8 m | Long rooms, driveways |
| 8–12mm | Narrow (~30°) | 12 m and beyond | Gates, long approaches, plates |
How resolution changes the math
Focal length does not work alone — resolution multiplies its reach. The identification distances above assume a 2MP camera; move to 4MP and every distance roughly grows, because more pixels pack more detail into the same angle. This is the professional lever: a 4MP camera on a 4mm lens can identify at distances a 2MP camera would need a 6mm lens to reach, while keeping the wider view. Choose focal length and resolution together — the lens sets the angle, the sensor sets how much detail survives across it — and read our resolution guide to balance the two.
When to stop calculating and go varifocal
Sometimes the exact ideal focal length is genuinely hard to know before the camera is on the wall — an odd distance, a scene you want to fine-tune, a mount you may need to re-frame later. That is exactly what a varifocal lens is for: instead of committing to one millimeter figure at purchase, you adjust the view on site until it frames perfectly. When a position resists a clean calculation, stop guessing between a 4mm and a 6mm and specify varifocal; our varifocal guide covers that decision in full.
A worked example: the villa gate
A common request we get: "put a camera on the gate so I can read the license plates of cars that stop." The distance from the mount to the stopping point turns out to be about eight meters. A 2.8mm wide lens there captures the whole street beautifully — and renders every plate as an unreadable smudge, because at eight meters a wide angle simply does not put enough pixels on the plate. The correct answer is a 6mm lens on a 4MP camera, which narrows the view to the drive but reads the plate cleanly at that exact distance. Same gate, same budget, two completely different outcomes — decided entirely by the millimeter number nobody thinks about until the footage disappoints.
The lesson repeats at every position: the distance and the goal choose the lens, and a five-minute measurement before ordering prevents the most common disappointment in surveillance — a camera that saw the incident but could not identify anyone in it.
Frequently asked questions
Is a wider lens always better value?
No — a wide lens that "sees everything" often identifies no one, because it spreads pixels too thin at distance. Match the angle to what the camera must prove, not to how much it can technically fit in frame.
What focal length for a shop entrance?
For a typical two-to-three-meter doorway, 2.8mm frames it and identifies faces comfortably. A wider storefront or a deeper shop steps up to 4mm; only long approaches need 6mm or more.
Does higher mm mean better zoom like a PTZ?
A fixed high-mm lens is permanently zoomed to one narrow view; a PTZ zooms live and moves. High focal length gives reach at a fixed angle, not the active tracking a PTZ provides.
Can I change a camera's focal length later?
Only on a varifocal or interchangeable-lens camera. A fixed-lens camera is locked to its angle, so choose the millimeters carefully before buying — or specify varifocal where the future is uncertain.
Related guides
- Varifocal vs fixed lens cameras
- 2MP vs 4MP vs 8MP: which resolution you actually need
- How to choose security cameras for your home
- PTZ vs fixed cameras: when do you need pan-tilt-zoom
Get the millimeters right for every camera
Tell FastEgy each camera's distance and what it must prove — detect, recognize or identify — and our engineers return the exact focal length and resolution per position, from stocked lenses with local warranty. Start at the contact page or browse cameras by lens in the FastEgy store.