首先我们需要找到透镜的焦点。

First we must find the focus of a lens.

youdao

眼镜都有一个光学中心，也就是说通过透镜的焦点看事物是最清晰的。

Glasses have what's called an optical center, which means that objects appear clearest when viewed through the center of the lenses.

youdao

就是从透镜(或者主反射镜)到焦点的距离，通常单位是毫米(mm)。

This is the distance (usually in mm.) from the lens (or primary mirror) to the point where the telescope is in focus (focal point).

youdao