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

通常，由于不同的反射镜和透镜使用，整个车灯组装就必须替换在切换到HID灯。

Usually, due to the different reflectors and lenses used, the whole headlight assembly will have to be replaced during the switch to HID lamps.

youdao

光经过透镜或反射镜得以集中，然后通过光导管进入室内。

The light is concentrated using lenses or mirrors and is then transmitted to building interiors by 'pipes'.

youdao