if you have a look at the import_chan_file.tcl that ships with nuke you will find that the last colum is actually the vertical FOV which is converted into the focal length upon import like this:
($va / 2.0) / (tan( [lindex $line 7] / 2.0 / 180.0*3.14159))
where $va is vertical aperture and [lindex $line 7] is the last colum in the chan file so you can read it like this:
focal length = (vertical aperture/ 2.0) / (tan( vFOV / 2.0 / 180.0*3.14159))
this is the trigonometry needed to derive the adjacent leg of a perpendicular triangle from the degrees of it's source angle and the opposite leg of the triangle.
To make sure we are looking at a perpendicular triangle we need to split the camera's frustrum through the middle which is why both vertical aperture and and vFov are devided by 2.
the vFOV is in degrees so therefore is converted to radiants first (180.0*3.14159).
the tan function is also defined as "opposite leg" / "adjacent leg" in a perpendiculat triangle so we can shuffle the equation to get the adjacent leg whici represents the focal length:
yikes, this doesn't make too much sense without a scribble, I'll put one together and should be much easier to understand (need a coffee first though)...
