Untested, because I couldn't be bothered to decide how to best get the viewport dimensions into the camera export function. But I think one would have to replace

        fov = 360 / pi *
atan(12.75 / focal)<br></br>

with something like

       if width
> height:<br></br>
           f =
height / width<br></br>
       else:<br></br>
           f =
1<br></br>
       fov = 360 / pi * atan(f
* 12.75 / focal)<br></br>

Your version should be a pretty good approximation for the range of focal lengths and image formats most people would use, though.

Edit: that was answering Dizzi, obviously.

-- I'm not mad at you, just Westphalian.