Vignetting is the effect in photography or in optics wherein the brightness on the periphery of the photo or the field of view is reduced to emphasize the center of the field. This usually creates an attractive effect on photos. However, vignetting occurs in telescopes which causes an unpleasant effect because the field of view becomes limited.

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 [...]