void OnFocusAndExposeClicked (UIGestureRecognizer gestureRecognizer)
{
CGPoint devicePoint = ((AVCaptureVideoPreviewLayer)PreviewView.Layer).CaptureDevicePointOfInterestForPoint (gestureRecognizer.LocationInView (gestureRecognizer.View));
SetFocusAndMode (VideoDevice.FocusMode, VideoDevice.ExposureMode, devicePoint, true);
}