Appendix B

Using the lens calculator to find out how different camera lens combinations perform in face recognition applications

Calculation examples

A face has in this example a designated width of 16 cm. A face needs to be represented by 80 pixels for identification purposes. The lens calculator can be used to decide at what distance from the camera, 80 pixels represent 16 cm. We therefore need to run the calculation in the following manner.

VGA resolution

If the camera offers max VGA resolution, then the total width of the camera scene is 640 pixels. A face of 80 pixels represent 12,5% of that. So, in order to find the maximum scene width where a positive facial ID can be performed, you divide 16 cm/0,125 = 128 cm.
Enter the result, 1.28 m in the lens calculator width and automatically you will receive 0.96 m in the height window. Then enter the focal length of the lens at max zoom, for example 8 mm for AXIS 221. You then push "Calculate" camera distance and VOILA = max distance for Face ID = 2.1 m.

Megapixel resolution

To perform the same calculation for a MPx resolution camera of say 1280 pixels then divide 80/1280 = 6,25%. Then 16 cm (face) / 0,0625 = 256 cm.
Enter 2.56 m in the lens calculator width and automatically you will receive 1.92 m in the height window. Then enter the focal length of the lens at max zoom, for example 3,6 mm (fixed) for AXIS 207MW. You then push "Calculate" camera distance and VOILA = max distance for Face ID = 1.9 m.

Considerations

Do it yourself!

Lens calculator tool

Variation will occur dependent on if 80 pixels is the standard to be used (specified in Sweden) or 60 pixels which may be sufficient in daylight situations. Wider facial features and also be considered, but for top detail use 16 cm.

Remember that face width is one of many criteria that need to be fulfilled for face recognition. If the surrounding light is not sufficient or a person is not facing the camera, you will still be in doubt.

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