Camera Mounting Height and the Blind Spot Underneath It

Coverage on a floor plan is drawn from above, so it shows the horizontal spread and nothing else. That view is honest about where a camera points and silent about what happens directly underneath it.

Every wall or pole mount has a wedge of ground beneath it that falls below the bottom edge of the frame. On a plan that area sits inside the cone and looks covered. It is not.

This is the most common oversight on a high mount, and it has a predictable cause: somebody puts a camera up out of reach so it cannot be vandalised, aims it near level so it gets range down the car park, and then cannot see the door it is bolted above.

Three inputs, and only three.

• Mounting height. Higher means a bigger blind spot, all else equal. This is the one that gets changed for reasons that have nothing to do with coverage.
• Downward tilt. More tilt shrinks it. This is the cheapest fix and it is usually a bracket adjustment.
• Vertical field of view. A wider lens sees more sky and more ground, so a wide camera has a smaller blind spot than a telephoto one at the same height and tilt.

The bottom edge of the frame sits at tilt + (vertical angle ÷ 2) degrees below level. Where that ray meets the ground is where the camera starts seeing, and the distance from the wall to that point is the blind spot: height ÷ tan(that angle).

Spec sheets quote a horizontal angle, or more often a diagonal one. The vertical angle is the one that decides the blind spot, and it is where a common mistake creeps in.

The vertical angle is the horizontal angle scaled by the picture's aspect ratio — and that means the resolution, not the sensor format. A sensor format like "1/2.8 inch" names a width, not a shape. Most IP sensors are natively 4:3 and crop to 16:9 for their headline resolution, so the active area is whatever the pixel dimensions say it is.

Assume the sensor's shape and you get a vertical angle half again too tall. A 4mm lens on a 1/2.8 inch sensor at 2688×1520 has a 40.2° vertical angle; treat it as 4:3 and you get 51.8°. On a 3.5m mount tilted 15° that is the difference between a 5.0m blind spot and a 4.0m one — and the error always runs in the direction that makes the install look better than it is.

There is no single right answer, but there are useful defaults.

8 to 10 feet (2.5-3m) is the sweet spot for anything that needs to identify a person. High enough to be out of casual reach, low enough that faces are in frame rather than the tops of heads. Entrances, tills, receptions, doors.

12 to 16 feet (3.5-5m) suits general area coverage — car parks, yards, warehouse floors — where you want range and are willing to give up facial detail. Expect a real blind spot and plan for it.

Above 16 feet (5m) is perimeter and overview work. At this height you are looking at the tops of things. If somebody needs to be identifiable up here, you need a second camera lower down, not a bigger sensor.

The trap is height chosen for a non-coverage reason — vandalism, cable route, an existing bracket — and then never re-checked against what the camera can see.

Tilting down closes the blind spot. It also pulls the top of the frame down, which shortens the range, and past a point the camera is looking at the ground just in front of it.

A camera aimed level or above is watching the horizon: the top of its frame never comes down, so it sees the sky behind whatever it is aimed at. That is occasionally what you want on a long approach, and usually it means somebody set the bracket and walked away.

The practical method is to decide what the camera is for first. If it is there to identify people at a door, tilt until the door is comfortably inside the frame and accept the loss of range. If it is there to watch a yard, accept the blind spot and cover the near ground with something else.

All of this is arithmetic you can do before anyone leaves the office. Put in the lens, the sensor, the resolution, the mounting height and the tilt, and the numbers fall out — including the blind spot and the range at which the camera stops being able to identify somebody.

The field of view calculator does it, and draws the side view so the gap under the camera is visible rather than implied. The horizontal coverage still belongs on the plan: see field-of-view cones explained for that half.