Focal RatioThe focal ratio of an optical system is defined as the focal length divided by the aperture. So, a telescope with a focal length of 600mm and a 60mm aperture has an f-ratio or f-number, of f/10. Telescopes with smaller ("faster") f-ratios exhibit wider fields of view and lower magnifications for a given focal length of eyepiece. They're best suited for viewing extended deep-space objects, and require shorter exposures for astrophotography. F/6 or lower is considered "fast". Instruments with medium f-ratios of f/7 to f/10 excel for both low-and high-power applications, with the use of different eyepieces. High focal ratios of f/11 and up are ideal for planetary observations and splitting double stars, but yield lower image brightness for a given focal length of eyepiece and a narrower field. They also require lengthy exposure times for photography. updated: |