How to programatically disable the auto-focus of a webcam

IAMCameraControl *pCameraControl;
HRESULT hr;
hr = pFilter->QueryInterface(IID_IAMCameraControl, (void **)&pCameraControl);
if (hr == S_OK) {
  long defaultFocusValue;
  hr = pCameraControl->GetRange(CameraControl_Focus,
                                NULL, // min
                                NULL, // max
                                NULL, // minstep
                                &defaultFocusValue, // default
                                NULL); // capflags
  hr = pCameraControl->Set(CameraControl_Focus, // property
                           defaultFocusValue, // value
                           CameraControl_Flags_Manual);
}