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The correct (as in recommended) way is using a CameraSubscriber. Using this approach, a callback is triggered that provides both the Image and the CameraInfo simultaneously.

Example usage for instance here.

Often, a lazy model subscription model is used, that only subscribes to camera data data when a output topic is subscribed at least once. An example of that approach can be seen here in the rectify nodelet. It's a little bit harder to read/understand, though.