Telescope

Eye Relief

Photo by: Tamasflex Creative Commons The eye relief is an important concept in the use of equipments like a telescope, microscope, or binoculars. By definition, eye relief is the distance from the eye piece at which the eye can be placed to the entrance pupil of the eye. Instruments with short eye relief will require

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Focal Ratio

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

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