For areas where the focal length would go below?

Focal Length: 45mm from center of lens decreasing in focal length linearly by 2.1 millimeters per millimeter of radius. For areas where the focal length would go below 30mm use 30mm focal length instead. 遇到这种长句子 我就头晕了 老外什么意思呢 烦请高手帮忙 谢谢！ Focal Length是焦距，lens是透镜