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Focal length has same meaning for lenses and telescopes, ie 500mm focal length in a telescope will give exactly the same results as a 500mm lens.
What is to some extent confusing is that the photo industry always refers to lenses by focal length - eg a “500mm lens” whereas telescopes are generally referred to by aperture - eg “100mm refractor” means a scope of aperture 100mm which will be somewhat larger in comparison and may have a focal length anywhere between 500 and 1500mm.
Focal length = aperture x focal ratio.
Last edited by Wavytone; 09-07-2019 at 12:40 PM.
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