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Magnification equals telescope focal length divided by eyepiece focal length. A 1200mm scope with a 25mm eyepiece gives 48× magnification.
Magnification = Telescope FL / Eyepiece FLExit pupil is the diameter of the light beam leaving the eyepiece. It equals aperture divided by magnification. Pupils dilate to ~7mm dark-adapted; larger exit pupils waste light.
Exit pupil = Aperture / Magnification (mm)True field equals apparent field of the eyepiece divided by magnification. A 50° apparent field eyepiece at 100× gives 0.5° true field (about one lunar diameter).
True FOV = Apparent FOV / MagnificationUpdated: July 2026
1200mm FL / 25mm = 48× magnification. Exit pupil = 203mm / 48 = 4.2mm. With 50° AFOV eyepiece, true field = 1.04° — fits the full Moon.
Same scope with 5mm eyepiece: 240× magnification, 0.85mm exit pupil. Best for steady seeing on Jupiter and Saturn; too dim for deep-sky.
80mm f/5 refractor (400mm FL) with 24mm 68° eyepiece: 17× mag, 4.7mm exit pupil, 4° true field for sweeping star fields.
Low magnification provides wider field and brighter image for finding objects and large nebulae. Reserve high power for planets, double stars, and lunar detail in steady air.
Exit pupil larger than your dark-adapted eye (typically 5–7mm) wastes light at the eyepiece. At 100× on a 4-inch scope, exit pupil is 1mm — fine for planets but dim for faint galaxies.
Choosing the right eyepiece depends on magnification, exit pupil size, and true field of view. This calculator computes all three from your telescope focal length and eyepiece specifications so you can match power to target and sky conditions.