Eyepieces determine the magnification and field of view of a telescope. Different eyepieces are used to view different objects. Some objects, such as nebulae and star clusters, appear quite large and are best viewed at low magnifications (which give a wider field of view), whereas planets appear very small and are normally viewed with high-magnification eyepieces. One of the most common misconceptions in amateur astronomy is that magnification is the most important aspect of a telescope. In reality, the diameter (aperture) of a telescope determines its power and different eyepieces are used to get the best view of a given object. Often the best view is at a low magnification. Be sure to read the section on Understanding Magnification for more details.
How Eyepieces Work
Why are eyepieces even necessary? A telescope is an optical system that creates an image, just like a camera lens creates an image on film. In fact, placing a camera at the focus of a telescope will also capture an image, since the telescope becomes the camera lens. But, placing your eye at the focus point of a telescope does not produce an image. Why not? Because your eye is also an optical system. Your eye focuses light just like a telescope does, and it cannot focus on a real image such as that created by a telescope. It requires a virtual image, which is what an eyepiece creates.
Take a look at the diagram below. It shows that both a telescope and your eye focus light to a point. Placing an eyepiece at the focal point of a telescope then creates a light beam which is neither converging nor diverging. Your eye can then focus the light beam exiting the eyepiece.
Above: How an optical system consisting of a telescope, eyepiece, and eye creates a final image
The most important eyepiece characteristic is focal length. This is the number, in millimeters, written on the side of every eyepiece. It allows you to determine the magnification an eyepiece gives in combination with a given telescope. Magnification is determined simply by dividing the focal length of the telescope by the focal length of the eyepiece.
This means that a smaller number on an eyepiece gives a higher magnification. A 10mm eyepiece would provide twice as much magnification as a 20mm eyepiece. It also means that the same eyepiece gives different magnifications on different scopes. A 10mm eyepiece would be low power on a short-focal-length scope but high power on a long-focal-length scope. For example, on an 80mm short-focal-length refractor, a 10mm might only provide 40x magnification, but the same eyepiece on a 10" Schmidt-Cassegrain telescope would give 300x.
A typical eyepiece collection would include 3 eyepieces: one low power, one medium power, and one high power. The usual magnification range depends on the telescope, but for most scopes the normal range might be from 50x to 250x.
Above: Increasing the magnification makes the image larger, but the image gets dimmer and the field of view gets smaller
This is an important aspect of many eyepieces. Eye relief is the distance from the eyepiece to the observer's eye. The shorter this distance, the more difficult it can be to observe. Also, if the observer must wear eyeglasses, short-eye-relief eyepieces can be very difficult or impossible to use. Long-focal-length eyepieces (usually low power) inherently have long eye relief, so they do not need to be specially designed to increase eye relief. Short-focal-length eyepieces (usually high power), on the other hand, do not inherently have long eye relief and must be specially designed to make them easier to use.
Above: The eye relief of an eyepiece is the distance from the top lens in the eyepiece to the observer's eye
Above: Two short focal length eyepieces, one with normal eye relief and one specially designed with long eye relief. Note the difference in the size of the eye lens.
Field of View
The amount of sky seen through an eyepiece (called the true field of view) is determined by both the magnification and the eyepiece's apparent field of view. Apparent field of view is a design characteristic of an eyepiece design. Some eyepieces have narrow apparent fields and some have wide apparent fields. If the magnification is kept the same (i.e., the eyepieces have the same focal length), an eyepiece with a wider apparent field will have a wider true field.
Above: Changing the apparent field but not the magnification changes the field of view but not the object size
You can also change field of view by simply changing magnification. If the apparent field is kept the same, a lower power eyepiece will give a wider field of view. To view very large objects such as the Andromeda Galaxy or Pleiades star cluster, you need a very large field of view and hence a very low magnification. Field of view is very important for getting the best view.
Above: Increasing the magnification may not always result in a better view, especially if the object being viewed is very large
There are two standard sizes of telescope eyepieces. The sizes are determined by the diameter of the eyepiece barrel that fits into the telescope. The two standard sizes are 1.25" and 2". A third size, 0.965", is a smaller standard that is usually best to avoid (see below).
Almost all telescopes are designed to be used with 1.25" diameter eyepieces. Most telescopes will include at least one 1.25" eyepiece. Accessories such as Barlow lenses and filters are designed to thread into the barrel of these eyepieces, so such accessories are also distinguished by size. Good 1.25" eyepieces typically cost $40-200, although there are more and less expensive models.
The second standard size is the larger 2" diameter. Many telescopes will accept these eyepieces, though some telescopes will require an optional adapter. Not all telescopes work with 2" eyepieces. 2" eyepieces are wide-field, low-power eyepieces. Above a certain magnification (which depends on the design), 2" diameter barrels are not required, so not all wide-field eyepieces are 2"--some will still be 1.25" and this is not a disadvantage, just a function of the design. This is a common misconception. Accessories such as filters and Barlow lenses are designed for 2" eyepieces as well. 2" eyepieces typically cost $200-400, with some of the largest and highest quality eyepieces costing around $600. Some inexpensive models are also available for around $100, though these will obviously not have the features or quality of the more expensive eyepieces.
Above: A 2" wide-field eyepiece compared to a standard 1.25" eyepiece. Both are 26mm eyepieces.
The final eyepiece size is the one to avoid. 0.965" eyepieces are the standard size for "department store" telescopes. These inexpensive telescopes often frustrate new stargazers, and one of the primary reasons is that viewing through 0.965" eyepieces is all but impossible. Also, standard accessories such as Barlow lenses and filters are not normally available for these eyepieces. And you are usually stuck with the eyepieces that come with the scope since 0.965" eyepieces are rarely sold separately. The difference between a scope with 1.25" eyepieces and one with 0.965" eyepieces is usually the difference between a scope that ends up in the yard showing you the wonders of the universe and one that ends up in the closet collecting dust.