1. What resolution means
Resolution is the number of points in the picture. More points mean more detail — but only if the lens and light allow. An 8 MP camera with a poor lens gives a blurry picture with more pixels.
2. Recognition at 20 metres
At 20 metres, 4 MP usually recognises a face. 2 MP shows a person, but the face blurs. 8 MP adds detail that helps when you must read text or a plate.
3. At 40 metres
At 40 metres, 4 MP loses face precision. Here 8 MP and a matching lens start to pay off. A 12 mm lens narrows the angle and pulls the image closer.
4. At 80 metres
At 80 metres, no resolution works magic without the right lens and light. Thermal cameras and PTZ cameras with zoom are the tools for these distances. These are specialised needs.
5. When camera count beats megapixels
For covering space, two 4 MP cameras often beat one 8 MP. Coverage of corners and entrances makes a system usable; resolution makes detail readable.
6. Light decides
Resolution does not help in the dark without IR or light. A camera with IR range sees at night; without it the footage is darkness regardless of megapixels.
7. Recommendation
For entrances and tills, 4 MP is enough. For plates and distance, choose the lens before the resolution. Resolution is the last filter, not the first.
RULE OF THUMB: Lens before resolution. If the camera cannot see the distance, even 8 MP will not help.