What are Focal Length and Focal Ratio?
Focal length is a simple measure in millimeters of the path light takes before its focused in the eyepiece and is important to know when determining the magnification power. Focal ratio is the ratio of focal length to aperture. Long focal ratios (e.g. f/16) yield narrower fields of view, but with higher-contrast images desired by planetary observers. Shorter focal ratios (e.g. f/4) yield extremely wide fields and faster photographic speeds, but generally with a lower level of image corrections at the edge of the field. Most telescopes compromise at about f/10, a ratio that permits comfortably wide fields, reasonable photographic speed and very good image contrast.
