This is the distance (usually in mm.) from the lens (or primary mirror) to the point where the telescope is in focus (focal point).
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)。
2
This text is to compare Frensnel s zones plate with ordinary lens, on the axle point of light intension and focal distance as well as their difference in use.
当光通过菲涅耳半波带与普通透镜时，可发现在轴上点的光强、焦距以及用途上的不同。
3
A convex lens causes rays to converge on a single point, the focal point.