What is the 'correct' method for deriving vertical field of view?

[Njen]

So I've found three different ways to derive the vertical FOV (field of view) from the horizontal FOV, and each method gives me a slightly different number. So what I want to know, is which one is the correct method?

Code: [Select]

vFov1 = 2.0 * atan(tan($cam_render.fov / 2.0) / GetRendImageAspect())
vFov2 = atan(tan($cam_render.fov) * ((renderHeight as float) / renderWidth))
vFov3 = ($cam_render.fov / GetRendImageAspect())
So, if I have a 70mm camera, using a 36mm film back, and a resolution of '1920x803', the resulting values for each of the above are:

Code: [Select]

vFov1 = 12.2765
vFov2 = 12.9701
vFov3 = 12.0624