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Some information that I've learned on the first and second focal plane.

In a fixed power scope this would be irrelevant but in variable power scopes it can be important.

First focal plane reticles are installed in the optical path at a place forward of the ocular lense group. Second focal plane reticles are installed in the zoom mechanism of the ocular. It is said that there is essentially no point of impact change in a first focal plane scope as you change magnification. ...