问如何监控android相机当前的FocalLength？

EN

SurfaceHolder.Callback surfaceHolderListener = new SurfaceHolder.Callback() {

            public void surfaceCreated(SurfaceHolder holder) {
                camera=Camera.open();
            try {
                camera.setPreviewDisplay(previewHolder);
               camera.setDisplayOrientation(90);

            }
                catch (Throwable e){ }
            }
            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height)
            {

                Parameters params = camera.getParameters();
               Rect centralFocusArea = new Rect();
               centralFocusArea.set(holder.getSurfaceFrame().width()/2-10, holder.getSurfaceFrame().height()/2-10, holder.getSurfaceFrame().width()/2+10, holder.getSurfaceFrame().height()/2+10);

               ArrayList<Camera.Area> focusAreas = new ArrayList<Camera.Area>();
               focusAreas.add(new Camera.Area(centralFocusArea, 1000));

                //params.setPreviewSize(width, height);
                params.setPictureFormat(PixelFormat.JPEG);
                params.setFocusMode(Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO);
               // params.setFocusAreas(focusAreas);
                camera.setParameters(params);

                camera.startPreview();
                camera.autoFocus(new AutoFocusCallback() {

                    @Override
                    public void onAutoFocus(boolean success, Camera camera) {
                        // TODO Auto-generated method stub
                        Log.v("CAMERA", "FOCUS CHANGE:"+camera.getParameters().getFocalLength());
                        camera.getParameters().setFocusMode(Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO);

                    }
                });

            }
            public void surfaceDestroyed(SurfaceHolder arg0)
            {
                camera.stopPreview();
                camera.release();
            }

        };