QCameraViewfinderSettings is not working on mac osx

I want to capture image from camera and not save to disk.

so I subclassed QAbstractVideoSurface, then use the QCameraViewfinderSettings to set resolution of the camera,
this works fine on windows 7 32/64 bit , but not works on mac osx
with the same code.
On mac osx the camera use the biggest resolution, which ignored my setting.
And I think this is a bug/