The function of an eyepiece is to gather as much light from the viewed body in order to create a clear image. The type of eyepiece one needs depends on the f-ratio of the telescope. For instance, an f/10 telescope does not require any particular eyepiece in order to create a good image but a f/4 will need a specific kind.
When looking for an eyepiece, keep in mind the eye relief, or the distance from your eye to the eyepiece lens, as it determines if you can see the entire field of view. Eyeglass wearers, especially, need to have at least a 15mm eye relief or the outer portion of the viewing field will be cut off. Traditionally the eye relief was directly related to the focal length, but newer eyepieces differ from that method.
The eyepiece also determines the field of view. An eyepiece's apparent field is the angular diameter of the circle of light that the eye sees expressed in degrees. The apparent view, divided by the eyepiece's magnification determines the eyepiece's true field of view. The true field of an eyepiece is the actual amount of area which is seen when looking through the eyepiece that is attached to a telescope.
While magnification is not the most important aspect of choosing an eyepiece, it is something that needs to be considered. One needs to decide how much magnification they desire and the focal length that such an eyepiece will give them. If one already knows what magnification they would like, they can figure out the eyepiece's needed focal length by dividing the telescope’s focal length into it. Likewise the telescope's focal length can be divided by the eyepiece's focal length to determine the magnification.
The aperture of an eyepiece is the most important element. The aperture determines how much light will be taken into the eyepiece, and thus how clear the image will be. The aperture for an eyepiece should be determined by the power per inch and the exit pupil size. The exit pupil is the number of light rays which leave the eyepiece. The exit pupil of an eyepiece must be smaller than one's pupil and is calculated by dividing the telescope’s aperture in millimeters by the magnification. Or it can be determined by dividing the eyepiece’s focal length in millimeters by the telescope’s f-ratio.
There are six different types of lenses which one can purchase. They are the Huygenian, Kellner, Orthoscopic, Plossl, Erfle, and Ultrawide. The Hygenian eyepiece is obsolete now, and has an extremely short eye relief and small apparent field. The Kellner eyepiece is three-element and the least expensive lens appropriate for serious astronomers. The apparent field of a Kellner eyepiece is about 40 degrees and it provides sharp images at low to medium powers. The Orthoscopic is four-element and once considered the best lens available. Though the newer lenses have minimized its appeal because of its narrow field. The four-element Plossl is the most popular lens as of late. The apparent field of view for this eyepiece is 50 degrees and the image quality is excellent along with good eye relief. While the Plossl eyepiece was once considered a luxury lens, it is now more common. The Erfle is 5 of 6 elements, and while it offers a wide apparent field of 60 or 70 degrees, the image sharpness suffers at the edges with high magnifications. Ultrawides can be 7 or 8 element, and offer huge apparent fields (up to 85 degrees) which some people enjoy. The light transmission can be slightly diminished because of the numerous elements but the image quality is still fairly high. The Ultrawide eyepiece can have a very pricey tag however.
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