focal length

光学系统会聚或发散光线强度的度量,是光焦度的倒数。在单个凸透镜的简单情况下,它是沿光轴从透镜主平面到透镜焦点的距离(例如相机中放置胶片的位置)。对于更复杂或多镜片的光学系统,焦距不一定对应任何明显的可测量物理距离。；焦距；[化] 焦距

A measure of how strongly an optical system converges or diverges light, the inverse of optical power. In the simple case of a single convex lens, it is the distance along the optical axis from the principal plane of the lens to the lens's focal point (where, for instance, the film would be placed in a camera). For more complex or multi-element optical systems, focal length need not correspond to any obvious measurable physical distance.