Jump to content


Magnification and size?

Recommended Posts

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?

Link to comment
Share on other sites

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
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.