Because it is a division, not a property. Magnification is the focal length of the telescope divided by the focal length of the eyepiece, so it changes every time the eyepiece does — and any figure can be reached by fitting a poor enough one.
This is the single most consequential misunderstanding in the whole subject, and it is manufactured rather than accidental.
The arithmetic
Divide the focal length of the telescope by the focal length of the eyepiece. A telescope of one focal length with a 25 mm eyepiece and the same telescope with a 5 mm eyepiece give figures five times apart, and nothing about the telescope has changed.
Why the box prints a number anyway
Because a large number sells. Supplying a very short eyepiece of simple design produces a headline figure cheaply, and the same eyepiece produces an image too dim and too soft to use. The figure is real arithmetic applied to a bad choice.
What actually limits useful magnification
The aperture, which sets how much detail exists to be spread out, and the atmosphere, which on most nights blurs that detail before the optics run out. Past the ceiling, more magnification enlarges a blur.
Do the division yourself, for each eyepiece
Ignore it entirely on packaging, and calculate it yourself for each eyepiece you own or intend to buy. Two eyepieces that give nearly the same figure in your telescope are one eyepiece and a wasted purchase.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the distinction this page keeps: a magnification is not a property of a telescope. It is the focal length of the tube divided by the focal length of whichever eyepiece is fitted, so it changes every time the eyepiece does, and a figure printed on packaging is a statement about a supplied eyepiece. This page publishes both focal lengths and the aperture, and reproduces no packaging magnification as a specification.
The questions that keep arriving
So what should I look at instead?
The aperture, which decides what there is to magnify, and the focal length, which with your eyepieces decides what magnification you will actually use. This site publishes both and reproduces no packaging magnification.
Is there a useful upper limit?
A common rule of thumb puts it at roughly twice the aperture in millimetres, and the atmosphere usually caps it lower than that. Beyond the ceiling the image gets larger and no more detailed.
Last reviewed 17 September 2026