The front focal distance of an objective is the distance between the front surface of the objective (the vertex point of the first lens) and its front focal point. The back focal distance is the distance between the vertex of the last lens and the back focal point.
Focal distances should not be (but are sometimes) confused with focal lengths. The latter are measured from the principal points and are the same on both sides if one has the same refractive index on both sides. In contrast, the front and back focal distance can be quite different, even if there is air on both sides.
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