How many cameras do I need for my site?

The number follows from a single calculation

A camera covers a certain width — exactly as much as its pixels allow at the density you demand. That is one division, and nothing more.

• Width covered = horizontal pixel count divided by the pixel density required

A 4K camera has 3840 pixels across. At 125 px/m for “Recognise” that gives 30.7 metres. At Full HD with 1920 pixels it is 15.4 metres. At 250 px/m for “Identify” both figures halve.

That width is an upper limit and does not depend on the distance. Further away only means: narrower lens, same width.

For the forty-metre yard that means: at “Recognise” with 4K you need at least two cameras. At Full HD it is three. At “Identify” with 4K it is three as well.

Why the distance still matters

The width covered is fixed; the distance only decides which lens you need for it.

All four rows cover the same width. The only thing that changes is where the camera has to stand. That is good news for planning: you can pick the position by what the building allows and choose the angle of view to suit.

What to watch when you divide the site up

• Plan for overlap. Two cameras whose fields meet exactly leave a strip in between where both only see the edge. Ten to fifteen per cent overlap is usual.
• Divide by task, not by area. The yard may only need “Detect”, while the gate within it needs “Identify”. Two cameras with different jobs beat four identical ones.
• Think about obstruction. A trailer, a container or a parked lorry takes half the area away from a camera. Two viewing directions are worth more here than a higher resolution.

And the most common mistake: the panoramic camera in the middle of the yard that is supposed to see everything at once. It spreads its pixels evenly across 180 or 360 degrees, which leaves very few of them for any one spot. For an overview that is right. For a face at the gate you need something else beside it.

How to go about it

Walk the site once and write down the places where something actually happens. The gate, the loading bay, the staff car park, the door to the store. For each place note how wide the area is and what you want to see there.

Then work through each place on its own. In the cctvfinder you enter the distance, the mounting height and the object width, and you see straight away whether one camera covers that place or whether it will be two. What you end up with is a list with reasons, and every line in it can be checked.