In determining the magnifying power of any eyepiece you use in it, all it takes is a little math. The magnification of the telescope is just the ratio of the objective focal length to the eyepiece focal length. Just divide the focal length of the telescope by the focal length of the eyepiece.
M = Objective focal length / Eyepiece focal length
The units for the focal lengths both have to be the same units.
From the example above, two units are being presented: 50 centimeters & 20 millimeters.
Covert 50cm to mm = 500mm.
So, if you have a telescope that has a 500mm focal length and you are using a 25mm eyepiece, you will be getting 20X (500mm/25mm = 20X).
If you place a 25mm eyepiece in a telescope with a 1000mm focal length, you'll get 40X (1000mm/25mm = 40X). That is why the same eyepiece appears to behave differently in different telescopes!
Magnification can be a wonderful thing. It can reveal amazing planetary features, subtle details in nebula and galaxies, split tight double stars and make you feel like you are walking on the Moon.
However, high magnification also has some drawbacks. If you aren't using a telescope with a drive unit, you will constantly have to use your slow motion controls to "track" your subject as the Earth moves. This can be very frustrating when viewing.
Also, when you magnify, you are magnifying sky conditions. For example, if you are using 200X on a night with poor clarity and stability, you are magnifying those conditions two hundred times as well. Just remember your telescope's practical limits and how to determine its magnification - and enjoy pushing those limits on good nights! Read more at Telescope.com and AstronomyNotes.com to learn more.