Explore Scientific Eyepiece Guide


Calculating Magnification
Telescope Focal Length ÷ Eyepiece Focal Length = Magnification
Telescope Aperture ÷ Exit Pupil = Magnification

Calculating Exit Pupil
Eyepiece Focal Length ÷ Telescope f-ratio = Exit Pupil
Telescope Aperture ÷ Magnification = Exit Pupil

Calculating The Eyepiece Focal Length for Lowest Useful Magnification (LUM)
Telescope Aperture (mm) ÷ Fully Dilated Eye (mm) = LUM
Telescope Focal Length (mm) ÷ LUM = Eyepiece Focal Length

Calculating The Eyepiece Focal Length For Highest Useful Magnification (HUM)
Telescope Aperture (mm) X 3 = HUM or more conservative Telescope Aperture (inches) X 60 = HUM
Telescope Focal Length (mm) ÷ HUM = Eyepiece Focal Length

Calculating True Field of View (TFOV)
Eyepiece Apparent Field (AFOV) ÷ Magnification = TFOV
Eyepiece Field Stop ÷ Telescope Focal Length × 57.3° = TFOV


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