"Beyond this point, with shorter focal length lenses, the diameter of the self-focal zone can no longer increase because the geometrical focal diameter becomes smaller. That is to say, external focusing becomes dominant and there is practically no distinction between geometrical focusing and self-focusing. Therefore, the diameter of the laser beam is limited by the strong external focusing of the lens, as observed in insets of Fig. 3 where the plasma column diameter decreases for shorter focal length lens."

what is the difference between external focusing , geometrical focusing and self-focusing. I am confused with this three concepts. Who can tell me the difference between this three physics concpts?

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