Focal ratio is also commonly known as the f/number, f/ratio, f-stop, f-ratio, etc; and is notated simply as N. Focal ratio is the speed by which an optical system needs to be exposed to produce an image; and can be found by dividing the system?s focal length by its aperture. A lower focal ratio means the system will have lower magnification, a wider field of view, a brighter image, and a shorter exposure time needed to produce an image; hence they are also regarded as fast scopes or fast cameras.

Fast scopes are usually best used for wide field observation and deep space photography while slow scopes are best used in lunar, planetary, and binary star observations requiring higher magnification.

It is also best to use telescopes with a lower focal ratio when taking pictures of objects in the sky; for a clearer and brighter image.

Fast scopes are those with a ratio of f/4 to f/5 while slower scopes are those with f/11 to f/15 focal ratios. Telescopes with f/6 to f/10 focal ratios can function both as a slow or fast scope, but not very well at both extremes. They are often preferred by users who need a dual-function telescope.