Most quality telescopes require the use of an eyepiece as part of the magnification power of the telescope. If you have a telescope with a single eyepiece, you will want to investigate using one or two different ones for different purposes. The eyepiece telescope has this advantage of those that do not have a changeable eyepiece.
The eyepiece plays a major role in the magnification power of an eyepiece telescope. In order to calculate what the magnification would be with a different eyepiece you would divide the focal length by the eyepiece focal length. Since the focal length of an eyepiece telescope will remain constant, by changing the eyepiece you can change the power.
Not everyone changes an eyepiece to increase the magnification, especially if wanting to view areas that are closer or they want a wider field of view. One reason to reduce the magnification may be to create a finder lens for your telescope to be able to find an object. With a strong eyepiece telescope, finding the moon through the normal magnification could be like trying to find it while looking through a straw.
Effective Fields Of View Also Changes
The true field of view, that is what you can see as an undistorted image can also be adjusted by changing the eyepiece on an eyepiece telescope. The field stop on an eyepiece is the rotating ring that limits the field size. To find the true field of view, divide the field stop diameter by the telescope’s focal length and then multiply by 57.3.
In order to obtain lower power viewing of large objects your eyepiece should deliver the widest possible field of view. A one and a quarter inch eyepiece would have a field stop of 27mm maximum. The maximum field stop on a two-inch eyepiece is about 46 mm.
A general rule of thumb with an eyepiece telescope is that choosing an eyepiece with shorter focal lengths and larger apparent field of view, bringing brighter, clearer images of objects including fainter stars.
Two things to consider when looking at news eyepieces could be a Barlow lens, which simply is a magnifier, which adds magnification power to the eyepiece telescope, and what is called eye relief.
Eye relief is the space between the lens and your eye and should have enough room for you to be able to wear your glasses, if needed, as well as to keep your eyelashes from brushing against the lens.