Camera placement: the four shots every site needs

Camera systems are usually specified by count. "We need eight cameras." Eight cameras pointed at nothing in particular produce eight streams of nothing in particular, and the day something is stolen the footage shows a shoulder, a hood and a car with the plate out of frame.

Specify by shot instead. Four of them cover most sites, and the count follows.

Shot one: every way in, at face height

One camera per entrance, mounted so that a person walking in fills the frame from the waist up. That means low — two to two and a half metres — and pointing along the path of travel, not down at the top of a head. This is the camera that identifies somebody, and it is the one most often mounted too high on the argument that height stops tampering. A face at four metres is a hat.

Shot two: the approach, wide

A second camera covering the outside of the entrance from further back. It sees what the first cannot: the vehicle, the second person who waited outside, the direction they came from and left in. This is the camera that gives the identification camera its context, and on a lot of sites it is the one that catches the plate.

Shot three: the thing worth stealing

The stock room, the server rack, the till, the loading bay. Close enough that a hand in a drawer is a hand in a drawer. This is the shot a manager actually reviews, because it answers questions that are not about crime — who left the door open, when the delivery arrived, whether the fridge was closed.

Shot four: the cash and the counter

Where money changes hands, one camera looking at the transaction from the customer's side and slightly above. It protects staff as much as stock; most disputes at a counter are resolved by a minute of footage that shows both parties.

Things that ruin all four

• Backlighting. A camera facing a glass door at midday sees a silhouette. Face it away from the light or choose a camera with real wide dynamic range, and test it at the worst hour, not at installation time.
• Infra-red bounce. A camera with IR illumination mounted close to a wall or a ceiling tile floods itself with its own light at night. Move it or angle it.
• Resolution spent on width. A 4K camera covering a whole car park has fewer pixels on a face than a 2MP camera covering a doorway. Pixels per metre at the target is the number; the box's headline is not.
• Retention nobody checked. Sixteen cameras at high bitrate on a 2TB recorder keeps four days. The incident is discovered on day six. See the note on recorder storage.

Walk it before it is drilled

Stand where each camera will be, hold a phone at the mounting height, and look at what it sees. It costs an hour and it is the only survey that matters. A camera moved after installation is a second visit, a patched hole and a cable that is now too short.