Getting 2 values of focal length when finding Intrinsic camera matrix (F not Fx,Fy)?

Question

The following image is the example that was given in my computer vision class. Now I cant understand why we are getting 2 unique values of f. I can understand if mxf and myf are different, but shouldn't the focal length 'f' be the same?

Answer 1

If really single f wanted, it should be modeled in the camera model used in calibration. e.g. give the mx,my as constants to the camera model, and estimate the f.

However, perhaps the calibration process that obtained that K was not that way, but treated the two elements (K(0,0) and K(1,1)) independently.

In other words, mx and my were also estimated in the sense of dealing with the aspect ratio. The estimation result is not the same as the values of mx and my calculated from the sensor specifications.

This is why you got 2 values.

Answer 2

I believe you have an Fx and a Fy. This is so that the the matrix transforms on f can scale f in two directions x and y. IIRC this is why you get 2 f numbers