focal length of lens

例句：

1. 1. The camera has a focal length of 50mm.
   这台相机的焦距为50毫米。
2. 2.The photographer adjusted the lens to focus on the distant mountain.
   摄影师调整了镜头，将远处的山景聚焦。
3. 3. Lenses are used in cameras to change the focal length.
   镜头用于相机中改变焦距。