Sensors, lenses, WDR and low light
Sensor size, focal length and field of view, shutter speed, WDR, IR illumination and low-light performance.
- Relate sensor size, focal length and field of view
- Explain WDR and when it is needed
- Design for low light: illumination, shutter speed and noise
- Avoid the classic image-quality traps on site
Image sensor
Almost all modern surveillance cameras use CMOS sensors. Sensor size is quoted in legacy "inch" formats (for example 1/2.8", 1/1.8", 1/1.2") which describe a format, not a physical diagonal in inches. For the same resolution, a larger sensor has larger pixels, which collect more light and usually give better low-light performance and dynamic range.
Doubling resolution on the same sensor size makes each pixel smaller. A well-designed 4 MP camera can outperform an 8 MP camera with a small sensor at night. Compare cameras side by side in the real scene, or with independent test footage.
Lens: focal length and field of view
- Focal length (mm)
- Short focal length = wide angle, wide field of view (FOV), fewer pixels per metre at a distance. Long focal length = narrow FOV, more pixels on a distant target.
- Field of view
- Determined by focal length and sensor size together. Use the manufacturer's FOV or a design tool (e.g. AXIS Site Designer) rather than guessing.
- Varifocal / motorized zoom
- Adjustable focal length; remote zoom and focus let you set pixel density precisely during commissioning.
- Aperture (f-number)
- Lower f-number (e.g. f/1.4) lets in more light but gives a shallower depth of field.
- Iris control
- Fixed, DC-iris or P-Iris. P-Iris (precise iris) keeps the aperture in the range that gives the best sharpness and depth of field.
- IR-corrected lens
- Keeps focus when the camera switches between visible light and infrared at night.
Shutter speed and motion blur
In low light, cameras lengthen exposure time to gather more light, which blurs anything moving. A walking person needs a faster shutter than a static scene; moving vehicles need much faster still (LPR cameras commonly use very short exposures with IR). Most cameras let you set a maximum exposure time so you trade a little noise for sharp moving subjects. Decide based on the requirement: identifying a person walking through a door matters more than a clean but blurred image.
Wide dynamic range (WDR)
Dynamic range is the span between the darkest and brightest parts of a scene that the camera can capture at once. Entrances with daylight behind people, parking garage exits and windows all exceed what a single exposure can capture: you get silhouettes or blown-out windows.
- Multi-exposure WDR captures two or more exposures per frame and combines them. It handles strong backlight well but can show motion artefacts on fast objects.
- Digital WDR / tone mapping processes a single exposure to lift shadows; good for moderate contrast.
- WDR is often quoted in dB (e.g. 120 dB), but measurement methods differ between manufacturers, so numbers are not directly comparable.
- Axis' implementation is called Forensic WDR; other vendors have their own names. Test against the real door at the brightest time of day.
Low light and illumination
| Approach | How it works | Notes |
|---|---|---|
| Day/night with IR-cut filter | Removable IR-cut filter: colour by day; filter moves out at night for monochrome IR-sensitive imaging | Standard on most outdoor cameras |
| Built-in IR LEDs | Illuminate the scene with near-IR invisible (or faintly visible) to people | 850 nm LEDs show a faint red glow and reach further; 940 nm is covert but usually shorter range. Range claims are best-case. |
| External IR or white light | Separate illuminators for longer range or even light | White light adds colour and deterrence; check light-pollution rules |
| Low-light sensors | Large pixels, good noise reduction, colour in very low light | Axis calls its approach Lightfinder; many vendors have equivalents |
| Thermal | Heat imaging, no light needed | Detection, not identification |
Lux ratings on datasheets are measured under different conditions by each manufacturer (shutter speed, IRE level, AGC), so they are not comparable. Ask for night footage or test it.
Installation factors that ruin good cameras
- Pointing at the sun or bright lights; mount so light sources are behind or beside the camera.
- IR bouncing off nearby walls, eaves, or the dome bubble, washing out the image; adjust IR or position.
- Mounting too high: steep angles show the tops of heads, not faces. For identification at doors, aim for a shallow angle (roughly 15° or less below horizontal is a common rule of thumb).
- Spider webs and insects attracted by IR on bullets; plan maintenance.
- Focus set by day without checking at night under IR (use IR-corrected lenses).
Common mistakes
- Selecting cameras on megapixels and datasheet lux values alone.
- No WDR at a glass entrance with daylight behind people.
- Long exposure at night causing blurred faces and plates.
- Steep mounting angles that make identification impossible.
- Trusting IR range claims without testing in the actual scene.
Check your understanding
1.For the same sensor size, increasing resolution generally…
2.A lobby camera faces glass doors with bright daylight behind visitors. Which feature is most important?
3.Which IR wavelength is generally covert (no visible glow) but shorter range?
4.Why are datasheet lux ratings poor for comparing cameras?
5.What setting trades a little noise for sharper moving subjects at night?