将压玻璃弹片松开，再将挡光板插入。
Loosen the glass bracket , then plug the block in .
其实，挡光板及其上的开孔都是不必要的；单单反射镜本身就可以起小孔的作用。
Actually , the mask and openings are not really necessary ; the mirror alone could serve as aperture .
挡光板