I am using a webcam mounted on the ceiling to track some markers. I measured the area on the ground that the webcam is able to see, it is roughly 2.61m x 1.46m.
The output of the coordinates right in the corners of that boundary is however not 2.61 and 1.46, but rather 1.90 m and 1.10 meters. (Measured in the same coordinate space, so that is not the error).
I was curious how these values are obtained, so I noticed:
- 1.90/2.61 is roughly 0.73
- 1.10/1.46 is roughly 0.75.
The webcam I'm using is a Logitech Tessar / Carl Zeiss 2.0f/3.7mm, so the focal length should be 7.4mm, if I understand right.
If I attribute the error to measurement uncertainty (which is definitely there, I was not very accurate measuring the boundary frame), then this number looks suspiciously like the factor (in cm) by which the coordinate estimates are wrong.
Is this true, or is it just a coincidence in my case? I know close to nothing about cameras.