Calculating which CCTV camera you need

In many ways this article bridges our technical section and our knowledge bank section but there are some general pointers we can give which will be common to pretty much all CCTV systems . The first thing people usually do when calculating which CCTV camera to use is get the tape measure out or pace round an area. Onlookers will be reaching for their phones when they see you walking around in pronounced steps trying desperately to pace out an exact metre each time thinking you have finally lost the plot, but you know what you are doing.

When people call us on the phone to discuss their needs they usually start by describing each area of the property to be secured. "We have a front garden which is 23 metres long by 18 metres wide. We need to cover the gate in the middle and driveway to the side and I'd also like to cover my ornamental fountain which is over by the neighbour's wall"

The problem with CCTV is that you can't cover vast areas with a single camera. Or rather you can, but the trade off is no one area will get special attention in terms of detail recorded. Even with megapixel CCTV cameras once you widen the angle of what the camera sees recorded detail drops off dramatically as you move away from the camera. Think about it, you have a given number of pixels across the screen and as you move away from the camera the area the camera sees increases but that means the pixel density decreases. D1 resolution is 720 pixels wide by 576 pixels high. This means if your camera has a viewing angle of 90 degrees at 3 metres distance it will see a width of 6 metres, ie 120 pixels per linear metre. At 10 metres distance it will see a width of 20 metres, ie 36 pixels per linear metre, and at 20 metres distance it will see a width of 40 metres, a mere 18 pixels per linear metre. To put that into perspective just 3 pixels across the width of someone's face.

When the person looked around their garden they moved their head and focussed in on the areas of interest, the gate, the fountain and the driveway. A CCTV camera can't do this, it shares it's attention across the whole screen. Incidentally given how few pixels were filming that face there is no point in zooming in electronically, you won't see anything other than a pixelated mess. Neither will increasing the TVL (number of horizontal TV lines) of the camera help things either. The only way to get more detail in these areas is to physically zoom the camera in on them, but it can only look in one place at a time. So there is a trade off with CCTV cameras, you either have a wide angle of view or you have good levels of detail. You can't have both.

Think carefully about where you actually need to film and how much detail you need to capture when calculating which CCTV cameras to use. Don't try to machine gun the whole of an area with CCTV, it won't work. Instead try to think of your site in terms of specific target points where you can capture information. Obviously there might be some instances where all you need is an overall view, but don't kid yourself that you will be recording footage from which you can read number plates or identify people. Here in our offices we have a camera which films the front car parking area. It can't be seen from the office and so we just want to know as and when someone arrives. A separate camera films them close up as they drive in to record number plate details and so on.

As we are always saying the best solution is to use vari-focal cameras which can be zoomed in and out during installation allowing you to avoid wasting any pixels with too wide an angle of view or not filming a wide enough area with too telephoto (narrow angle of view) a camera.