How does reversing the orientation of a PCX lens affect the EFL and BFL in a setup?

Reversing the orientation of a Plano-Convex (PCX) Lens, pointing the convex side toward the source rather than the plano side, will maintain the location of the principle planes but will vary the Back Focal Length (BFL). Effective Focal Length (EFL) is measured from the principle plane so it is unchanged. BFL is a mechanical measurement and purely arbitrary. In the case of a PCX Lens, Edmund Optics® specifies BFL as the distance from the convex side to where incident collimated light comes to a focal point; if the lens is reversed, then this distance is now measured from the plano side to where the incident light focuses.

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