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The task of the eyepiece (technically called an ocular or ockulair in Europe where some of the leading telescope makers of a previous era resided) is to magnify the image collected by the telescope.
The most significant property of the eyepiece is its focal length - which appears to be 25mm, 10mm and 4mm in your case.
If you know the focal length (in mm) of your telescope it is then a simple matter to calculate the magnification - if you have an 8in (200mm) f10 telescope the focal length is 2000mm.
To get the magnification for each ocular divide its focal length into the focal length of the telescope. So the 25mm will divide x80 times into the focal length of 2000, so has a magnification effect of 80.
Similarily with the 10mm, the magnification is x200, and the 4mm x500.
Do you know the focal lenghth of your telescope?
I was recently asked - why? and I can't say I know, but it clearly has something to do with the ratio of the focal length of the ocular to the focal length of the telescope and the resulting image scale from using the two together.
The higher the magnification the greater the image scale and things will appear bigger.
Higher magnifiactions also magnify the atmospheric "seeing", which is the amount of atmospheric turbulence that you have to look through when using a telescope. With poor to average seeing, too high a magnification will only result in a blurred image, due to the distorting effect of the atmosphere.
It can also be due to the limitation of your telescope. If you have a very small telescope, say 60mm or 114mm, increasing he magnification does not always give you a better image, as the ability of a small telescope to resolve small details is limited. But the main effect on limiting the magnification you can use is seeing. It is unlikely you will ever use the 4mm eyepiece.
Generally you would start observing with a 25mm, and maybe increase the magnification depending on what you are observing and the seeing conditions.
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