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)。
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。
A convex lens causes rays to converge on a single point, the focal point.
会聚透镜使光线会聚于一点，即焦点。
The parallel light rays of the prnicipal axis that approach a convex lens can converge to the focal point.
只有在近轴光线条件下，平行光通过凸透镜才可近似聚焦于一点。
Two kinds of convex lens which make parallel light rays converge to the focal point are given.
本文给出两种能使平行光严格聚焦于一点的凸透镜。
A discussion is given of the method of determining the position of the focal point when the measured lens is with primary astigmatism. The measuring results are also included.
被测物镜有初级象散时确定焦点位置的方法，并给出了测量结果。
A concave lens causes rays to diverge as though they are coming from a focal point.
发散透镜使光线分开，就好象从焦点上发出来的那样。
A concave lens causes rays to diverge as though they are coming from a focal point.
发散透镜使光线分开，就好象从焦点上发出来的那样。
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