What exactly do you need to recognise?
There are three separate jobs, and they can't be swapped for one another. Detection answers whether something moved. Recognition answers whether it's someone you know. Identification answers who it is, reliably enough to report to the police. Each job needs a different pixel count on the target, so the answer to this question decides the camera — not the label on the box.
Write down the job for every position before anyone sends you a quote: entrance — identification, walkway — recognition, yard — detection. That one sheet of paper saves more than any discount.
Distance and lens: why 4 MP at 40 m shows nothing
Resolution is spread across the width of the scene. The wider the lens, the fewer pixels land on a face. A 4 MP camera with a wide lens covering a 20 m yard puts a person into a few dozen pixels — enough to see that someone is there, not enough to say who.
Pixels per metre of scene width. The exact figure is set on site, tape measure in hand.
Rule of thumb
- Detection
- ~25 px/m
- Recognition
- ~125 px/m
- Identification
- ~250 px/m
Night: IR range, WDR and street lighting
The IR (infrared) range on a spec sheet is measured in clean air, in a dark room. Rain, fog, dust and a white wall near the lens all cut it short. A strong light behind the target turns a face into a silhouette — that's what WDR (Wide Dynamic Range) is for, and having it at an entrance matters more than another two megapixels.
Where street lighting already exists, use it: a camera working in that light captures colour, and colour describes clothing and vehicles. Infrared only gives you a grey shape.
How many days of footage do you actually need?
Long enough for someone to notice the loss and ask for the footage. In a shop that's usually 15 to 30 days. In a warehouse it's one stocktake cycle, because a shortfall surfaces when the count is done, not when it happens.
Retention is a storage question, and storage is cheaper than a second call-out. As a rough figure, 6 GB per camera per day at 4 MP, 15 fps and H.265 (a newer compression standard, roughly half the size of H.264 for the same picture) — nine cameras over thirty days comes to about 1.6 TB.
Power and cabling: PoE, 12 VDC, 24 VAC
PoE (Power over Ethernet) carries data and power over a single cable, which is why it wins in new installations. An existing RG59 coax run isn't automatically scrap: an AHD/TVI system over it reaches 4 MP without opening a single wall, which saves a part of the budget no one plans for.
Ask one question about power: what happens when it cuts out. A camera without a UPS (uninterruptible power supply) records the moment the lights went out and nothing after it.
Who's watching, and from where
This is the question that decides whether a system has an owner. If the answer is “we'll check if something happens,” you've bought an archive, not surveillance. If more than one site is involved, a VMS (Video Management Software) on a single login is worth more than logging into four separate recorders.
Decide, at the same time, who is allowed to open the archive, and write it down. Named accounts and a logged viewing history protect you on the day someone asks who watched what.
When a cheap kit makes sense, and when it doesn't
It makes sense when the goal is deterrence at a single spot and no one will ever open the archive: a garage, a storage room, a weekend house with nothing in it. It doesn't make sense when the footage needs to work as evidence, when the site operates at night, or when more than one person needs access.
The honest test is one sentence: if this footage had to go to the police tomorrow, would a face be visible on it? If the answer is no, the money spent on it is already lost.