无焦系统（afocal system）也称为远焦系统或焦外系统，是指对光束没有净发散或净聚焦的光学系统，也就是说光学系统的等效焦距为无限大，这种光学系统可以用一对光学组件组成，在其距离等于其组件焦距和（d = f₁+f₂）时即有此特性。像光学望远镜就是无焦系统，进入光学系统的光是来自无穷远处，而成像也是在无穷远处（光线为平行光）。

无焦系统不会将平行光聚焦或是发散，但会改变光束的宽度，增加望远镜放大倍数。其放大倍数如下

在雷射光学中会用到无焦系统，例如扩束器、红外线及前视红外线系统、相机变焦镜头、像teleside转换器等望远镜头配件以及结合相机及望远镜的无焦摄影。

In optics an afocal system (a system without focus) is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length. This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's focal length (d = f₁+f₂). A simple example of an afocal optical system is an optical telescope imaging a star, the light entering the system is at infinity and the image it forms is at infinity (the light is collimated). Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam, increasing magnification. The magnification of such a telescope is given by