When viewing extended objects, the apparent brightness seen in the eyepiece is a function only of aperture and magnification, it is not related to focal ratio.
当观察扩展光源时，目镜中所看到的亮度只跟孔径和放大率有关，它跟焦比并没有关系。
2
If you know the focal ratio but not the focal length, you can calculate it: focal length is the aperture (in mm) times the focal ratio.