What is called the optical center, the main plane, the side optical axis of a thin lens?

The main plane of the lens – the plane in which the limiting surfaces of the lens lie and the optical center passing through its optical center – is located at the point of intersection of the main optical plane of the lens and the main optical axis of the lens perpendicular to this plane. A straight line passing through the optical center of the lens and not coinciding with the main optical axis is called a secondary optical axis.

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