ardrone_autonomy IMU and camera data fusion

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I use the message_filters::sync::ApproximateTime to get the IMU and camera data, however, since the camera data is 30fps, the interval between two frames should be near 0.033ms or its times. However, the time stamp I got from the camera is vary differently. I don't know whether it's normal or not. Besides, should I be fine to just use the time stamp or I need to consider the off-time? How should I do that? Thanks