Rotate Camera without affecting Layer Boundaries

Dear Community,

how to describe my confusion?
It´s a HD 720p Composition.
i´m trying to rotate a cameras Z-Position without rotating my video 3D-layer itself.
If i rotate the camera, my video layer behaves like if i would rotate the Z-Position of my Video Layer itself and the background of my composition becomes visible.

I thought i could rotate the camera and you just see the rotation not affecting the layer itself.
means the layer stays in place and just the camera is making the layer turn upside down. I cannot zoom into the video to make the background disappear otherwise i have a loss in image quality

don´t know if you understand my question.

thanx in advance.
cheers,
Paul