But I've never actually heard of a game allowing specification of the vertical FOV. Usually FOV simply means horizontal FOV.
If a game is vert -, then FOV means horizontal FOV.
If a game is hor +, then FOV means vertical FOV.
And in this case, changing the FOV slider does change both the horizontal and vertical FOV.
I explained this already. The horizontal FOV is dynamically calculated, based on the vertical FOV and the aspect ratio. If you change the vertical FOV, the horizontal FOV will automatically change as well.
But you're confident that the ultimate, resulting horizontal FOV will be set correctly and automatically by the game engine based on the resolution aspect ratio chosen?
Basically, except it's not even the game engine that takes care of that. The Source engine only "knows" about the vertical FOV. Direct3D takes care of calculating the horizontal FOV.
And you can test this by setting the FOV slider to any value you like, and then taking comparison shots yourself at two different resolutions. You will see that when the FOV slider is the same but the aspect ratio is different, the vertical FOV will be the same but the horizontal FOV will be different but correct for both pictures.
A lot of the Vert - games that we have solutions to get into Hor + behavior use the Vertical fov. What happens is the increased vertical viewing area brings out the extra Hor stuff on the sides. It is just a different path to the same solution(Changing the Hor fov).
Actually, you've got it backwards. If a game is vert -, then FOV is horizontal FOV, and vertical FOV is calculated dynamically. By increasing the horizontal FOV to what it should be, the vertical FOV is dynamically increased back to what it used to be.