var sc_invisible=1; Can be measured in Degrees or Minutes of Arc (arcminutes). The magnification factor M B of a Barlow lens depends on its position relative to the focal plane of the telescope’s objective lens and the focal plane of the eyepiece. Field of View Calculator Test different telescope, camera & eyepiece combinations. 1000 / 10 = 100. On average, this maximum decreases as we age; while the average teenager's pupil can dilate to 7.5mm, the average 78 year old's eye can only reach about 5mm. //--> Enter the eyepiece size in millimeters; also enter the lens' focal ratio. (And for further information, also see our How Telescopes Work and Choosing a Telescope pages.). The larger the f Ratio of the telescope, the further the Exit Pupil moves out from the eyepiece; this holds true if the telescope's focal length is "artificially" extended by using a Barlow lens. Indeed, our seeing often creates a resolution limit well above this level. I understand that the calculation for a single barlow is simply: Magnification = - d / f + 1where d is the distance to the barlow lens (back, center?) I want to find a 1.25 Barlow that will give about 1.65x to 1.7x when placed about 25mm to 30mm in front of the focal plane. Notes: 1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x. current page:     More Resources>       About Telescopes>         'Scope Computer, If you or your club wants to translate this page to your native tongue, the NAA will be happy to host the translated version of the page.