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Telescope focal length and magnification explained
Magnification is not a property of the eyepiece alone: it comes from the telescope's focal length divided by the eyepiece's focal length, so the same eyepiece gives different powers on different scopes.
The magnification formula
Magnification equals the telescope focal length divided by the eyepiece focal length. A telescope with a 1200 millimetre focal length used with a 10 millimetre eyepiece gives 120 times, because 1200 divided by 10 is 120. The same 10 millimetre eyepiece on a 600 millimetre telescope gives only 60 times.
This is why a magnification number printed on an eyepiece box is meaningless on its own. The eyepiece contributes only its own focal length; the telescope supplies the other half of the sum.
Shorter eyepiece means more power
A shorter eyepiece focal length gives higher magnification. On the same 1200 millimetre telescope, a 25 millimetre eyepiece gives 48 times, a 10 millimetre gives 120 times, and a 6 millimetre gives 200 times. Building a set of a few eyepieces across this range covers low, medium and high power.
A Barlow lens multiplies the effective magnification, typically by two or three, by extending the light path, which is a way to reach higher powers without buying very short eyepieces.
The useful upper limit
There is a ceiling set by aperture, not by the eyepiece. A common rule of thumb is a maximum useful magnification of about fifty times the aperture in inches, or roughly two times the aperture in millimetres. An 8 inch, 203 millimetre telescope tops out near 400 times in good conditions.
Push past that limit and the image grows larger but dimmer and softer, because you are magnifying the blur from the atmosphere and the optics rather than revealing new detail. On many nights the sky itself limits useful power well below the theoretical maximum.
Lowest useful magnification
There is also a practical low end. Below a certain power the beam of light leaving the eyepiece, the exit pupil, grows wider than the pupil of your eye, so light is wasted. This sets a sensible lowest power and is closely tied to exit pupil, which is covered in its own guide.
See the magnifications your telescope reaches
Open the Fit My Telescope gear finderPick your telescope and eyepieces to see the resulting powers.Find telescope eyepiece set on eBayaffiliateRelated
Sources: Celestron telescope basics documentation ยท Sky-Watcher telescope specifications