All eyepieces have a focal length and it is very important to know what it is and what it means. Being made of usually several lenses, the eyepiece will have a focal plane.

That's where the eyepiece focuses on and examines the image formed by a telescope, for example. Usually the focal length is printed right on the barrel of the eyepiece in millimeters. The importance of this is that you can determine the magnification of your telescope by knowing the scope's focal length and the eyepiece focal length.

Say you have a telescope that has a focal length of 500 mm. this would also likely be somewhere on the scope or in the instruction manual. The magnification of the telescope will be the focal length of the telescope divided by the focal length of the eyepiece. So if you have a 500 mm focal length telescope and use a 10 mm eyepiece, the magnification is 500/10=50X.

So you can see the shorter the focal length of the eyepiece for a given telescope, the more power or magnification. But there are drawbacks to magnification. With higher magnification you get a narrower field of view, compared to lower magnification. Also, for a given scope there is a limit to the amount of magnification it will handle. Too much magnification and the image gets blurry or soft. Magnification is not the most important number when using a telescope. There's more to it, but I hope this helps

Typically,you do not want to exceed 2x per mm of your telescopes diameter or the images will be blurry;for example,your telescope's objective diameter is 80mm,The maximum magnification should not exceed 160x

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