Focal Length of Spotting Scopes
The focal length (f.l.) of a telescope is simply the distance between the main lens or mirror and the point where the light rays from the image come sharply into view. The eyepiece also has a focal length and the combination of these two focal lengths result in the magnification of the image (mag. = f.l. of objective divided by f.l. of eyepiece). Focal length is often expressed in another term, focal ratio, which is the ratio of the aperture diameter to the focal length of the objective lens. For example, the focal ratio of a spotting scope with a 90mm aperture with a1000mm focal length would be 1/11 (90mm/1000mm) or more commonly written as f/11. This relationship may also be referred to as the photographic speed, f/stop, or f/number. As a general rule, longer focal length telescopes have higher f/numbers and those with a shorter focal length have smaller ones. A telescope f/number will usually range from f/4 to f/15 and is an important consideration if you plan to use the telescope as a telephoto lens as smaller numbers will give you shorter photographic exposure time.
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