Two quotes land on the desk. One offers 4K cameras throughout; the other proposes a mix of resolutions and lenses and costs less. Instinct says more megapixels must be better. In practice, the camera that identifies a face at the door and the one that only shows a blur can have the same resolution, because what matters is how many of those pixels land on the subject, and that depends on the lens, the distance and where the camera is mounted.
Pixels per metre, not megapixels
The useful measure of a camera view is pixel density: how many pixels fall on each metre of the scene at the distance where the subject will be. The surveillance industry describes this with four levels, usually abbreviated DORI. Detection, seeing that a person or vehicle is present, needs only a few dozen pixels per metre. Observation, following what someone is doing, needs more. Recognition, telling that it is a known person, needs roughly 125 pixels per metre. Identification, producing an image that would stand up as evidence of who a stranger is, needs roughly 250 pixels per metre.
Those figures explain the two quotes. A wide-angle 4K camera covering a whole car park spreads its pixels over a huge area and may not reach recognition level anywhere except the nearest few metres. A modest camera with a longer lens pointed at the entrance door can hold identification level exactly where it counts. Good design starts by asking what each camera is for, then works back to the resolution and lens that deliver that density at that distance. That is the thinking behind our commercial CCTV and surveillance designs.
Focal length and field of view
The lens sets the field of view. A short focal length gives a wide angle that takes in a large area but shrinks everything in it; a longer focal length narrows the view and enlarges the subject. Doubling the focal length roughly halves the width of the scene and doubles the pixel density on whatever remains in frame. There is no free lunch: a camera cannot simultaneously watch an entire lobby and identify faces at the far wall.
Fixed versus varifocal
A fixed lens is set at the factory and is the right choice where the scene is predictable, such as a corridor or a till. A varifocal lens can be adjusted during commissioning, usually with motorised zoom and focus from the recorder, so the installer can fine-tune the framing once the camera is on the wall. It costs a little more and repays that on exterior mounts and anywhere the ideal framing is not obvious from a drawing.
Light, night and difficult scenes
Resolution is measured in a bright showroom; footage is reviewed after an incident at two in the morning. Low-light performance depends on the sensor size, the lens aperture and how the camera processes the image, and small pixels on a high-resolution sensor gather less light each. That is why a lower-resolution camera with a larger sensor often produces a cleaner night image than a higher-resolution one.
Wide dynamic range, usually shortened to WDR, is the camera’s ability to handle a bright background and a dark foreground in the same frame, the classic problem of a doorway with daylight behind whoever is walking in. Without good WDR the person is a silhouette. Infrared illumination lets the camera see in complete darkness, but the range printed on the datasheet is measured under ideal conditions, and a subject at the edge of that range appears as a faint grey shape rather than an identifiable face. Plan for identification well inside the stated range, and remember that infrared shows monochrome, so colour details are lost at night unless the area is lit.
Height and angle
Where the camera goes matters as much as what it is. Cameras mounted high to keep them out of reach look down steeply, and a steep angle shows the top of a head, a hat brim or a hood rather than a face. For identification at a door, a camera mounted a little above head height, looking almost horizontally at the approach, produces usable images; the same camera on a soffit three storeys up produces a record that somebody was there and nothing more. Overview cameras can sit high; identification cameras need to come down to where the faces are.
Angle also affects motion. A camera looking straight down a corridor sees people walking towards it, holding them in frame for several seconds; a camera looking across the corridor sees a brief sideways blur. Turning a camera along the direction of travel often does more for evidence quality than upgrading its resolution.
Common questions
Is a 4K security camera worth it?
Where a single camera must cover a wide area and still hold detail at distance, yes, because the extra pixels can be spread across the scene without dropping below the density you need. Where the camera watches a narrow, close view, a lower resolution with the right lens does the same job for less storage and bandwidth. The answer depends on the scene, which is why mixed designs are so common.
What resolution do I need to identify a face?
Think in pixels per metre rather than resolution. Identification of a stranger generally needs about 250 pixels across each metre of the scene at the point where the person will be. A camera can reach that with a modest resolution and a longer lens close to a door, or with a high resolution and a wide lens over a short distance. Distance, lens and mounting height decide it together.
Do I need varifocal lenses on every camera?
No. Fixed lenses are fine where the framing is predictable, and they are simpler and slightly less expensive. Varifocal lenses earn their place outdoors, at long or uncertain distances, and anywhere the ideal view cannot be settled from a drawing. Being able to adjust zoom and focus from the recorder after mounting saves a return visit and lets the installer hit the pixel density the scene demands.
How far can an infrared camera really see?
Less than the number on the box suggests. The stated range is where infrared can still detect something in ideal conditions, not where a face is identifiable. Reflective surfaces, fog, snow and rain all shorten it, and subjects near the camera can be overexposed while distant ones vanish. Treat the stated range as a detection distance and plan identification points at a fraction of it, or add lighting.
If you are comparing camera proposals and want each view judged on what it will actually show, we are glad to walk the site with you. See how we design commercial camera systems, or get in touch and tell us which doors and areas matter most.
