巴罗镜（Barlow lens）的的名称来自创造者英国工程师彼得·巴洛（Peter Barlow）。巴洛发展出一系列可以插入其他光学系统的透镜组。当这些透镜被置入后，可以明显的察觉出原系统的焦比被有效的增加了。

它在天文学的用法是将巴罗镜直接安装在目镜的前端，由于巴罗镜的分流而有效的延长主镜的焦距。由于望远镜的放大率是主镜焦距除以目镜焦距的商值，因此可以增加影像的放大倍率。

天文学上使用的巴罗镜放大倍率是固定的，最常用的是2X或3X，但也有可以调整的。可调整的巴罗镜视增加一个可以调整的延伸**变它与目镜的距离，来增加放大的倍数。

The Barlow lens, named after Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, decreases the effective focal length of an optical system as perceived by all components that are after it in the system. The practical result is that inserting a Barlow lens magnifies the image. A real barlow lens is not a single glass element, because that would generate chromatic aberration, and spherical aberration if the lens is not aspheric. More common configurations use three or more elements, for achromatic correction or apochromatic correction and higher image quality.