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Magnification Guide

What power are you viewing at?  Use this handy chart to help!

Magnification = Telescope Focal Length / Eyepiece Focal Length

Focal Length


2" f/15




5" f/10


6" f/8


7" f/15


8" f/10


10" f/5


10" f/6


12" f/10


40mm 19X 29X 32X 30X 67X 51X 32X 38X 76X
32mm 24X 36X 40X 38X 83X 63X 40X 48X 95X
26mm 29X 44X 49X 47X 102X 78X 49X 59X 117X
20mm 38X 57X 63X 61X 133X 102X 63X 76X 152X
14mm 54X 82X 91X 87X 190X 145X 91X 109X 218X
10mm 76X 114X 127X 122X 267X 203X 127X 152X 305X
8mm 95X 143X 159X 152X 333X 254X 159X 191X 381X
5mm 152X 229X 254X 244X 533X 406X 254X 305X 610X

Values have been rounded to the nearest number.


Example of a plot that can help your decision in which eyepiece to purchase:

(You can download the source Excel Spreadsheet here.)

For this 8" Schmidt-Cassegrain, anything over 400X is optical quality limited.  Anything over 55 mm will cause vignetting around the image.  This plot shows there aren't any gaps in magnification for these sample eyepieces, and maybe that there is a close duplicate between the 9mm and 9.7 mm eyepieces that could be eliminated and not leave the observer wanting for something in between.


Introduction to the Night Sky - Part I

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Introduction to the Night Sky - Part II

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