How can I test if camera calibration was optimal?

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"But how can I know that camera calibtation was optimal? "

That's a tricky question. A good rmse (well below a pixel for most cameras) is a good hint, but not the perfect solution as a calibration with a single pattern could also give a good rmse but a bad calibration in total.

"And how to visualize calibration effect?"

You can create a grid of points across your undistorted image and use cv::undistortPoints to get their original poses. Draw lines between these two points and you get an understanding of the distortion effect.