较长的焦距
Longer focal length
以上为机器翻译结果，长、整句建议使用 人工翻译 。
该方法借助一个焦距较长的凸透镜与被测凹透镜组成无焦系统，可以使准直管出射光束的张角发生改变；
Firstly, a telescope system is made up with a concave lens and a long-focus convex lens, by which the angle of the parallel beam from the collimator can be change.
youdao
应用推荐
模块上移
模块下移
不移动