A traditional measuring method of focal length of thin concave lens was introduced and a new method occasionally found in experiment was designed in this paper. The new formula was derived and verified by experiment. Both methods were compared with each other and analyzed the different phenomenon.%首先介绍了一种经典物距像距法测定薄凹透镜焦距的实验方法，并由交换透镜位置出现的偶然现象设计新的测定方法，推导出该方法的测量公式并进行可行性检验，分析了两种方法的优劣，最后对两种实验不同的现象进行分析，由理论验证实验现象。
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