Camera Fov Question

[agartha]

I'm working on a camera importer for Maya and the source cameras provide only projection/view matrices and FOV (field of view) values.

In Maya FOV/AOV is a derived attribute of the horizontal film aperture and focal length rather than an attribute of the camera. Maya uses the following equation to determine FOV:

CODE
proc float AEcalculateFOV( string $focalStr, string $horStr )
{
    float $focal = getAttr $focalStr;
    float $aperture = getAttr $horStr;

    float $fov = (0.5 * $aperture) / ($focal * 0.03937);
    $fov = 2.0 * atan ($fov);
    $fov = 57.29578 * $fov;

    return $fov;
}

Maya also provides a solution to getting the focal length given FOV and horizontal film aperture:

CODE
global proc AEadjustFocal( string $focalStr, string $horStr )
{
    float $fov = floatSliderGrp -q -value fovGrp;
    float $aperture = getAttr $horStr;
    float $focal = tan (0.00872665 * $fov);
    $focal = (0.5 * $aperture) / ($focal * 0.03937);

    setAttr $focalStr $focal;
}

If all I know about a camera is its FOV, How do I calculate both the horizontal film aperture and focal length?

Thanks in advance.