CCTV Camera Coverage: How Much Area Can a Camera Cover?

Coverage is often described as the area a camera can see, but being visible in the picture is not the same as being usefully covered. A person in the far corner of a wide scene may be present in the footage and still be too small to tell anything from.

It helps to separate four ideas:

• Field of view: how much of the scene the camera takes in, from side to side and top to bottom.
• Useful detail: how much a person looking at the footage can actually make out in a particular part of the scene.
• Viewing distance: how far from the camera the subject is. The same camera shows a nearby subject larger and a distant one smaller.
• Target area: the specific place the camera is there to watch, such as a doorway, a till point or a gate, as opposed to everything it happens to see.

Good coverage means the target areas are in view, at a distance where the detail is good enough for the job the camera is there to do.

The main factor is the lens. A lens with a shorter focal length takes in a wider scene; a longer focal length narrows the view and magnifies what is in it. This is why a wide lens suits an open yard or a room seen from a corner, and a narrower lens suits a single entrance or a long approach.

Resolution works differently. More pixels do not widen the view, but they give more detail inside whatever the lens captures, which can make a distant subject easier to make out. Resolution and lens are chosen together: a high-resolution camera with a very wide lens can still show distant subjects only as small shapes.

The scene itself matters as much as the camera. The same camera gives a very different result in a narrow corridor, an open car park or a room with shelving, because the geometry of the space decides how far the view can run before it meets a wall or an obstruction, and how much of the picture is taken up by something that does not need watching, such as sky or a blank wall.

As a subject moves away from a camera it takes up fewer pixels, so it shows less detail. That is true of every camera. What changes from camera to camera is how quickly detail falls away, which depends on the lens and the resolution.

For that reason, a figure for how far a camera can see is incomplete unless it says what the camera needs to show at that distance. Spotting that something is moving in the distance is a much lower bar than recognising a person or reading a vehicle number plate, and the same camera may manage the first and not the second at the same distance.

When planning, start from the task rather than the distance. Decide what must be identifiable and where, then work out what lens and camera position can deliver that.

A wide field of view is useful for situational awareness: seeing who is in a room, where people are moving and what is happening across a space. It is a less reliable way to capture fine detail far from the camera.

A narrower field of view gives up some of the scene in return for more detail in what remains. It is better suited to a defined point of interest, such as an entrance, a gate or a counter.

Where a camera is fixed changes what it can usefully see as much as the lens does.

• Angle. A camera looking along a route, such as a corridor or the length of a fence, sees people for longer than one looking across it. Looking straight at a subject generally shows more than looking steeply down on it.
• Height. A higher position can see over some obstructions and is harder to reach, but a camera that is too high or too steeply angled may see the tops of heads more than faces. The right balance depends on the site.
• Obstructions. Walls, pillars, shelving, vehicles, signs and planting all block a view. Some obstructions are only there at certain times, such as parked vehicles, stock or seasonal growth.
• Overlapping views. Two cameras whose views partly overlap can cover an area from two directions, so that something blocked from one position can still be seen from the other.

If a camera is positioned so that it also captures areas beyond the property, such as neighbouring land or a public space, that can raise privacy and legal considerations that should be understood before installation.

A blind spot is any area that matters but is not in view, or is in view without enough detail to be useful. They are usually the result of planning rather than faulty equipment.

Some practical ways to reduce them:

• Mark the priority areas first, and plan from those outwards rather than trying to see everything.
• Look for blind spots directly under and close to a camera. A camera aimed far into a scene can miss the ground near its own mounting position.
• Check around corners, behind obstructions and across changes of level, where views are most often interrupted.
• Allow for things that move, such as doors, vehicles and stock.
• Let views overlap where a failure of one camera, or an obstruction, would leave a priority area unwatched.

There is no standard number of cameras for a given size of building or site. The count follows from the layout and what needs to be seen, not from floor area alone. The main factors are:

• how many entrances and exits there are, and how they are approached;
• the layout of the space, including corridors, corners and changes of level;
• how much detail is needed in each area;
• what obstructs the views;
• which areas are the priority, and which only need general awareness; and
• how much overlap is wanted between cameras.

A small site with several entrances and awkward corners can need more cameras than a larger open one. This is why a camera count suggested without seeing the site should be treated as a starting point.

Coverage in the dark is not the same as coverage in daylight. How much a camera can usefully show at night depends on the light available, including street and building lighting, and on the camera’s own ability to cope with low light. Distance, detail and the position of any lighting all affect it, so a camera that covers an area well in the day may cover less of it at night.

It is better to plan night-time coverage deliberately, with the lighting in mind, than to assume a camera will perform in the dark as it does in daylight.

Specifications and rules of thumb can only take planning so far. The deciding factors are the real geometry of the site: where the entrances are, what the sight lines are, where cameras can be fixed and powered, and what is in the way. These are best checked on site, ideally at the times of day when conditions are most demanding.

A useful plan usually covers:

• what each camera is there to show, and how much detail that needs;
• where each camera can realistically be fixed;
• what each view will include, and what it will leave out; and
• how the cameras together deal with blind spots and low light.

Camera coverage is not one number. It is the result of the lens, the resolution, the position and the surroundings, judged against what the system needs to show. A wide view and a narrow view each do a different job, and good planning matches them to the areas that matter.

If you are planning a system and want the coverage assessed against your site, see our CCTV system installation in Manchester, or get in touch to talk it through.