Technology
Why megapixels say nothing about picture quality
2 min readcctvfinder
You are standing in front of two cameras, comparing. One has four megapixels, the other eight. So you take the one with eight; that makes sense. Except that this number says almost nothing about whether you will recognise a face in the end. Why that is takes three lines to show.
Two cameras, the same eight megapixels
Take two cameras that are identical on paper: both 4K, both 3840 pixels across, both pointed at the same spot 18 metres away. The only difference is the lens. One looks wide at 126°, the other narrow at 42°.
| Wide angle 126° | Telephoto 42° | |
|---|---|---|
| Resolution | 3840 × 2160 | 3840 × 2160 |
| Image width at 18 m | around 72 m | around 14 m |
| Pixel density at the target | around 53 px/m | around 274 px/m |
| Level reached | Observe | Identify |
The same eight megapixels, and two quality levels between them. One camera is good for “someone was there”, the other for “that was this person”.
The calculation in three lines
Pixel density follows from the width a camera covers at the target distance and the number of pixels it spreads across that width.
- Image width at the target = 2 × slant distance × tan(field of view ÷ 2)
- Pixel density = horizontal pixels ÷ image width
- Level reached = the highest one whose requirement the pixel density meets
What is remarkable is what does NOT appear in that calculation: the megapixel figure. What counts is the pixel count across the width alone, because pixel density is defined horizontally.
A panoramic camera at 5120 × 1440 has fewer megapixels than 4K. Horizontally it still delivers a third more pixels.
What counts in the data sheet instead
- Horizontal field of view in degrees, as the manufacturer measures it. This is the most important number.
- Horizontal resolution in pixels, not in megapixels.
- For varifocal lenses, both ends of the zoom range. They often differ by a factor of seven in pixel density.
- The lens speed, if the site is to be watched at night. Pixels that disappear into noise are not pixels.
Compare on those four figures and you are comparing cameras. Compare on megapixels and you are comparing packaging.
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