Hi,
When parameter
approx_sync is true (default true for depth/color and false for stereo),
ApproximateTime policy is used, otherwise it is
ExactTime policy. When Exact, timestamps for all input messages should be exactly the same for the callback to be called. When Approximate, you can also tune the
queue_size parameter. Increase
queue_size if some topics to be synchronized are published faster than others. For example, if images are published at 5 Hz and odometry at 150 Hz, you need a
queue_size of size at least 30. With very high odometry framerate, you may want to use the odom TF interface by setting "odom_frame_id" parameter (TF will be used to get the odometry pose, so /odom topic will not be synchronized, only the images).
cheers