in the Perspective Node ( Perspective (Transform Offcenter) [nid:5703] ), there is the FOV inlet. I since forever wonder what kind of number has to go in there. Is it like a throw ratio, so width of view related to distance? Or is there a relation to an angle, or to focal length? But focal length would be related to some kind of theoretical ‘sensor size’…

It’s the angle of your perspective, like it’s explained in the link antokhio sent.

If you imagine your camera like that:

Everything that’s between the Near Plane and the Far Plane is what you’re going to see using this camera. The values you pass for Near Plane and Far Plane is the distance of them respectively to the camera.

The FOV will be the angle you see in the image - here’s the Y value of the FOV, the horizontal one is the X value.

I believe the angle is in cycles, as everything in vvvv is - hence 0.25 would be a right angle. Not sure about that though, don’t have vvvv here to try.

Does that make sense for you?
Maybe you can build a helppatch that would’ve helped you for this node.