For beginners, this is often regarded as a very important factor when buying a telescope. It is, however, not the most important characteristic of a telescope, but rather plays a subordinate role. Much more important are the light intensity and how stable the telescope is held by its mount.
Basically a telescope, depending on the curvature of the mirror or lens, focuses incoming light at a focal point. The focal length means a certain magnification factor is already achieved. But one needs an additional eyepiece to view the resulting image. This basically works like a magnifying glass which enlarges the image accordingly.
The achievable magnification depends on the ratio of the focal length of the telescope to the focal length of the eyepiece. If divide the focal length of the telescope (fob) by the focal length of the eyepiece (fok) you get the magnification achieved.
If, for example, you use a telescope with a 1000mm focal length and a 5mm eyepiece, you will get a magnification of 200 times. Theoretically, one could increase the magnification ever higher. But, as this is related to the aperture of the telescope, there is a limit to sensible magnifications. An important role is played by the exit pupil, the beam that exits the eyepiece and enters the eye. More on this in a moment.
There are not only limits set on the maximum magnification set by the aperture size, but also on the minimum magnification. The exit pupil should never be greater than seven millimetres. This is usually also the maximum aperture achievable by the pupil of the eye; and only then in absolute darkness at night. If you now divide the telescope aperture by the diameter of the maximum aperture of the pupil, it will give you the minimum useful magnification.
At this magnification, a beam of seven millimetres diameter will still pass completely through the eyepiece and into the eye. Now if the exit pupil were to be even larger at a lower magnification, then the remaining light will be lost as the eye’s pupil starts to block the outer region of the light bundle.
For a telescope with a 200mm aperture, the minimum useful magnification would hence be about 28X. If the aperture of the telescope were larger, then the minimum magnification would also be higher; and with a smaller telescope lower accordingly.
The normal magnification of a telescope is approximately that of its aperture in millimetres. The normal magnification will give an exit pupil of about 1mm. This magnification lets an observer use the achievable resolution of the telescope. This means that considerably more detail, e.g. on planets, is visible.
A telescope with a 100mm aperture would have a normal magnification of100X and a 200mm telescope 200X. The maximum useful magnification can be calculated using this rule of thumb:
The exit pupil is here reduced to 0.5 mm:telescope aperture / maximum
Magnification = 0.5. If higher magnifications than this are used, then the image will become dim and blurry.
The normal magnification is always usable. Problems often arise at the maximum magnification due to the Earth's atmosphere, meaning it is not always possible to go to the maximum limit. The reason here due to different layers of hot and cold air that lie over each other in the atmosphere. This phenomenon is also called the ‘seeing’ and is often dependent on meteorological aspects. Through the telescope, bad seeing makes itself felt as stars twinkling. When you select a high magnification, the air turbulence cells are also enlarged. If you have ever observed Jupiter shortly after it rises in the east, you may have noticed that the image appears somewhat unsteady and blurry. But wait for a couple of hours and observe again when it has risen higher and you will find that the image is much more sharp and steady. The seeing is always worse on the horizon and it makes sense to use lower magnifications when observing here.
There is a general rule which one should consider when choosing the magnification for observing astronomical objects: For nebulae and other large objects it should be rather low (up to 100X) and for planets be somewhat higher (more than 150X).