How to convert pixel relative focal lengths to mm. Ive been playing around with the Intel Realsense R200 Camera.
I am able to exact focal length in x and y i.e. fx and fy,
but only "up to scale relative to pixel width and height, respectively"

At the moment I dont see any way of querying the pixel dimensions in mm from the API nor can I find them in the tech specs. 

So my question is: How can I get the fx and fy in mm? 
One way I can think of is to take a picture of my checkerboard pattern and count the number of pixels and divide by the size in mm.

Is there a better/more elegant way? It seems kind of..imprecise.

Best,
Marc
There is only one focal length, $F$. It is the distance between the focal point and the image plane. 

The numbers you have, are actually not lengths. They are calculated by $f_x=F\cdot s_x $ and $f_y=F\cdot s_y $  where $s_x$ and $s_y$  are the size of your image in pixels. To get the focal length in mm as you want, simply divide $f_x$ with the width (in pixels) of your image.