Megapixels, sensor, focal length, IR range, IP67, WDR: here is what each line of a camera spec sheet changes in the picture. The full system (recorder, hard drive, cabling) is covered in the guide to choosing a video surveillance system, and standalone models in the guide to choosing a Wi-Fi camera.
HD analogue or IP: two families, the same image criteria
HD analogue cameras (HDCVI from Dahua, Turbo HD or HDTVI from Hikvision) send video over coaxial cable to a DVR or XVR. They now go up to 8 MP 4K, with a microphone and colour lighting (see the HDVI cameras). IP cameras use an RJ45 network cable, often with PoE. Just make sure the camera and the recorder speak the same signal.
Resolution: count the pixels on the target
Megapixels do not tell you how far away a face can be recognised, and labels such as 2K, 3K or 4K differ between brands. Go by the horizontal pixel count.
| Label on the spec sheet | Usual format | Horizontal pixels |
|---|---|---|
| 2 MP, Full HD 1080p | 1920 x 1080 | 1920 |
| 3 MP, sold as 2K by Tapo and Imou | 2304 x 1296 | 2304 |
| 4 MP, QHD | 2560 x 1440 | 2560 |
| 5 MP, often sold as 3K | 2880 x 1620, or 2960 x 1665 on Hikvision | 2880 to 2960 |
| 8 MP, 4K | 3840 x 2160 | 3840 |
The EN 62676-4 standard sets pixel density thresholds, known as DORI: 25 pixels per metre to detect someone, 62 to observe, 125 to recognise a known person, 250 to identify a stranger. With a 2.8 mm lens (about 100° horizontal), the scene is almost 12 m wide at 5 m from the camera. A 2 MP camera puts 160 pixels per metre on it (recognition), an 8 MP about 320 (identification). At 10 m the 2 MP drops to 80 pixels per metre: you see someone walk past, but not their face.
For a face at the gate, a longer lens on a 4 or 5 MP camera often does better than a wide-angle 8 MP.
Sensor and aperture: what matters at night
Sensor size is written as a fraction of an inch, usually 1/2.7 or 1/2.8. On the same sensor, more megapixels means smaller pixels that collect less light, so an 8 MP on a 1/2.7 inch sensor is often noisier at night than a 4 MP on the same sensor. For aperture, the lower the number (F1.6 rather than F2.0), the more light gets in. Starlight refers to this sensitivity, with no added lighting.
Lens and field of view
On common sensors a 2.8 mm lens covers about 100° horizontally (99° to 107° on Dahua spec sheets). A 3.6 mm lens covers less but makes the subject larger. Some sheets give the diagonal angle, which is always wider: 121° for the 2.8 mm version of the IMOU Bullet 2E.
- Fixed 2.8 mm: yard, shop, room. Overall view, little detail beyond a few metres.
- Fixed 3.6 to 6 mm: corridor, driveway, gate. Narrower view, larger face.
- Motorised varifocal: the angle is set remotely after installation. The Dahua HDCVI 5 MP 2.7 to 12 mm varifocal goes from 107° to 34°.
- Pan and tilt or dual lens: most Wi-Fi models rotate but zoom digitally, which enlarges the picture without adding detail. The dual-lens Tapo C245D pairs a 122° wide angle with a 65° telephoto.
Night vision: infrared, colour or hybrid
The stated range is how far the light reaches, not an identification distance: at 80 m even a 4K camera only detects.
| Technology | Night image | What to check |
|---|---|---|
| Infrared (IR, EXIR, Smart IR) | Black and white, invisible light | Range from 20 to 80 m depending on model, glare behind glass |
| White LEDs (Full Color, ColorVu) | Colour, visible light all night | Can bother neighbours, attracts insects |
| Hybrid (Smart Dual Light, Smart Hybrid Light) | IR by default, white light when needed | Default mode and what triggers the switch |
| Starlight sensor alone | Colour as long as some light remains | Depends on street lighting or a lamp |
Three examples: the Dahua 2 MP Full Color with 20 m LEDs films in colour all night, the Hikvision Turbo HD 3K Smart Hybrid Light with microphone combines 30 m IR and 20 m white light, and the Dahua HDCVI 4K with 80 m infrared covers a large yard in black and white.
Weatherproofing and housing
IP65 protects against dust and low-pressure water jets. IP66 withstands powerful jets, which covers driving rain. IP67 also survives temporary immersion. The rating does not cover the cable connection, which needs a junction box. Within reach of hands, look for the IK10 impact rating rather than a bare anti-vandal label. Even under an awning, choose an outdoor security camera.
Audio
On an HD analogue camera, sound from the microphone travels over the coax: the recorder must support it or the sound is lost. On wired cameras the advertised audio is usually a microphone only. To talk back you need a speaker and two-way audio, common on Wi-Fi models.
Smart detection
Basic motion detection reacts to branches, headlights, shadows and cats. Person and vehicle classification, sold as WizSense by Dahua and AcuSense by Hikvision, filters those alerts. In an analogue system it is often handled by the XVR, so check both spec sheets. The Dahua 4 MP Smart Dual Light PoE IP camera does human detection on its own. Face recognition is a different feature, found in higher ranges.
The other lines on the sheet
- WDR: True WDR, in dB (120 dB for example), comes from the sensor. DWDR is a software correction, weaker against a glass door in full sun.
- 3D-DNR, BLC, HLC: noise reduction, backlight and headlight compensation.
- Frames per second: 25 fps is enough for surveillance. Some 4K models drop to 15 fps.
- H.265: much lighter than H.264. The H.265+ variants cut bitrate further on quiet scenes, if the recorder supports them.
- Power: 12 V for analogue, PoE or 12 V for IP, mains or battery for Wi-Fi.
Checks before buying
- Measure the distance to the spot where a face must be seen and work out the pixels per metre.
- Confirm the recorder accepts the technology (HDCVI, TVI, IP) and the resolution.
- Pick the night mode based on the neighbours and existing lighting.
- Outdoors, aim for IP66 at least and plan a sealed junction box.
- If sound matters, check the microphone and audio support on the recorder.
Common mistakes
- Putting an infrared camera behind a window: the IR reflects and the picture turns white.
- Expecting a wide-angle 8 MP camera to read a number plate at 15 m.
- Mounting too high: at 5 m you film the tops of heads.
- Pointing at the setting sun or a lamp: even with WDR, the subject becomes a silhouette.
Models in stock are on the security cameras page, along with a selection of indoor security cameras.
