定义

有效焦距（EFL）是指主点到焦点的距离。

后焦距（BFL）是指最后一片透镜的顶点到第二焦点的距离。

前焦距（FFL）是指第一片透镜的表面到第一焦点的距离。

English Expression

focal length ( f )

Definition

The effective focal length (EFL) is the distance from the principal point to the focal point.

The back focal length (BFL) is the distance from the vertex of the last lens to the second focal point.

The front focal length (FFL) is the distance from the first lens surface to the first focal point.

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