What is focal ratio?
Focal ratio is the focal length divided by the aperture. In photography, “focal ratio”, or “f-stop”, indicates the ability of the lens to gather light for exposure. In telescopes, it is the combination of aperture and magnification that determines how bright an image appears. A fast lens with a small aperture cannot gather sufficient light from a faint, distant nebula, but a large enough aperture lens of a high focal ratio may provide excellent viewing of dim, distant objects..
