Understanding video surveillance resolution can be surprisingly difficult and complex. While the word 'resolution' seems self-explanatory, its use in surveillance is far from it. We will explain 5 critical elements:
What resolution traditionally means – seeing details - and the constraints of this approach
What resolution usually means in surveillance – pixels – and the limits of using this metric
How sensor and stream resolutions may vary
How compression impacts resolution greatly
What limits resolution's value
Resolution – Seeing Details
In normal English and general usage, resolution means the ability to resolve details – to see or make them out. For example, can you read the lowest line on an eye chart? Can the camera clearly display multiple lines side by side on a monitor? etc. It is a performance metric focusing on results.
For example, in IPVM testing, the human eye is roughly equal to a 10MP resolution camera.
Historically, video surveillance used a similar test chart approach. Analog camera resolution was measured with line counts, literally the camera's ability to display more lines side by side in a given area on a monitor.
If you could see more lines, it meant you could see more real world details
facial features, characters, license plates, etc.
You can do that. For example, we did just such a chart based test, and found 720p cameras to be roughly equal to 500 'lines', 1080p roughly 900 'lines', etc. as shown in the image below:
However, manufacturers almost always use pixel count instead of line count for resolution.
Moreover, lines counted was always done in perfectly even lighting conditions. However, with direct sunlight or low light, the resolving power would change, falling significantly. Even more challenging, some cameras fared far worse in these challenging lighting conditions than others.
While this approaches measures performance, it only does so in the most ideal, and often unrepresentative, conditions.