Magnification (Power) and Using Eyepieces
Understanding how power, or magnification, is calculated when using a telescope will require the understanding of a relationship between two independent optical systems - the telescope itself and the eyepiece you are using. To understand this we must first understand the term Focal Length.
Calculating Magnification (power)
There are practical lower and upper limits of power for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60mm telescope (2.4" aperture) is 142x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Be very cautious of manufacturers who advertise a 375 or 750 power telescope which is only 60mm in aperture, as this is false and misleading. Many department store brand telescopes know that customers are not informed how telescopes operate. These manufacturers of telescopes market their products to the misconception that magnification is the most important feature on a telescope.
Most of your observing will be done with lower powers 6 to 25 times the aperture of the telescope (in inches). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.
There is also a lower limit of power which is between 3 to 4 times the aperture of the telescope at night. During the day the lower limit is about 8 to 10 times the aperture. Powers lower than this are not useful with most telescope and a dark spot may appear in the center of the eyepiece in a Catadioptric or Newtonian Reflector telescope due to the secondary or diagonal mirrors shadow.
Most telescopes will come with 1 or more eyepieces. It is good to have a wide selection of eyepieces for various types of observation. To truly get the most out of your telescope it is a good idea to have a wide selection of eyepieces. You will find that most telescopes will come with a 25mm (sometimes 20 or 26 depending on the telescope) because this is one of the most common eyepieces focal lengths to use to get the optimum magnification and field of view from your telescope. It is good to spread out your selection of eyepieces. A good selection would be a 5mm - 10mm - 15mm - 25mm - 32mm as an example. You will find that many companies offer kits of eyepieces that will give you a wide selection in one package.
Here is a list of manufacturers that make exceptionally high quality eyepieces. Although these eyepieces are amazing in quality - some of the prices for an eyepiece alone could purchase a complete telescope!
Here is a list of manufacturers that make some very good eyepieces at a good price.