Hi susantini
FOV normally narrows down when passing the camera feed through the preview output. Setting the previewSize() to what your network expects (usually 300, 300 - which is 1:1 aspect) will crop your feed. By setting setPreviewKeepAspectRatio(False), you should get the full FOV of the video. More info on maximizing FOV here.
Hope this helps,
Jaka