The CCTV industry is pushing Mega Pixel (MP) High Definition (HD) as the big thing in cameras but is it really replacing the analogue camera.
The following takes a technical view at some of the prose and cons based the comparison of a 35X analogue PTZ compared to a fixed position 5MP camera looking across a typical sports stadium.
Effective resolution of digital zoom
My research has highlighted to me that digital zoom of a mega-pixel camera also has the inverse square law applied.
This means that for every 2X zoom, you divide the pixel count by 4. for 3X zoom, by 9. And the killer, an 8X zoom is divided by 64. Therefore if you have a 5MP camera at each zoom step, you will end up with the following effective pixel count.
| Digital Zoom | 0X | 2X | 4X | 6X | 8X |
| Pixels H x V | 2592 x 1944 | 1296 x 972 | 648 x 486 | 432 x 324 | 324 x 243 |
| Relative MP Size | 5MP | 1.2MP | 0.32MP | 0.14MP | 0.08MP |
| Equivalent lens | 16mm | 32mm | 64mm | 96mm | 128mm |
| Near 720P HD | Same as VGA | Same as CIF |
Using a CCTV 3D drawing software, this gives the following results based on the following cameras and zoom ratios
| 5MP ½” with 16mm lens set to full resolution | 35X PTZ set to 4CIF |
|
0X digital zoom |
10X optical zoom |
|
4X digital zoom |
15X optical zoom |
|
8X digital zoom |
35X optical zoom |
5MP ½” with 16mm lens 8X digital zoom resolution
35X PTZ set to 4CIF 35X optical zoom resolution
5MP ½” with 16mm lens 0X digital zoom
35X PTZ set to 4CIF 10X optical zoom
5MP ½” with 16mm lens 4X digital zoom
35X PTZ set to 4CIF 15X optical zoom
5MP ½” with 16mm lens 6X digital zoom
35X PTZ set to 4CIF 30X optical zoom
Lighting
The inverse square law applies to lighting in CCTV, which states that every time you double the distance from the camera to the object of interest, the light reaching the camera is reduced to ¼.
This means for a camera looking across the oval some 170+ metres, the lux level required on the person is going to need to be very bright. Looking are night time games on television, the rear 2/3rd rows of seating are quite dark. I do not know what the actual lux level is and this may be worth looking into prior to installing the cameras.
Conclusion
As illustrated, the results of a 5MP compared to a 35X PTZ are very much in favour of the PTZ.
The effective resolution of a 8X digital zoom is CIF and the PTZ can produce the same level of zoom at 4CIF.
Based on these results, I would recommend either install 3 or 4 PTZ cameras, which will give improved angles of view and not suffer from as much sun glare in the evening matches from at least 2 of the cameras.
However once HD PTZ cameras become a common product, this will give the advantage on the analogue PTZ plus the resolution of the MP camera.
Example angles of view using 3 35X PTZ cameras
Example angles of view using 3 35X PTZ cameras showing the same person from different angles
