# Magnification and size?

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As a complete beginner I'm posting here a lot recently, but I have a 70mm refractor with two magnifications, 20mm (45x) and 10mm (90x) and I wonder what rule is there to determine what size eyepiece will give me a certain magnification. Like....does this mean a 5mm eyepiece will give me 180x, going by this trend? Or am I getting the magnification rules totally wrong?

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Your telescope's focal length is 900mm and to get the magnification you divide that by the focal length of the eyepiece.

900/20 = 45

900/10 = 90

900/5 = 180

You would probably find a 5mm eyepiece too high and not comfortable to use. A common rule of thumb for maximum magnification is 2 time aperture in millimetres, i.e. x140, which you would get with a 6mm eyepiece, though even that might prove too high for good views - you'd just need to try it and see. The two eyepieces you've got give sensible magnifications for the scope and should prove very useful.

Edited by acey
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Thanks, i get it now you've made it make sense

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