Explain the difference between power and vertex power from basic definitions of optics, and analyze the measuring principle and error of focimeter.

从光学的基础定义出发，阐述光焦度和顶焦度的差别，并对焦度计的测量原理和误差进行正确分析。

Objective：To confirm that the lensometer adjusted by the special lenses group for contact lenses cant express the back vertex power of contact lenses.

证实采用接触镜专用标准透镜组标定的焦度计不能准确测定被测角膜接触镜的顶焦度。