Eyepieces Eyepieces are critical parts of any optical system. It is not unusual to have several eyepieces for a given telescope. In general, lower power eyepieces give wider fields of view, and higher powers give narrower fields. Good eyepieces, especially ones with very wide fields, can be very expensive. Focal lengths are almost always given in millimeters.
Modern eyepieces are much more than a simple lens.
More elaborate coatings make an eyepiece more expensive. Most binoculars, and some telescopes, can use a simple photographic tripod, but telescopes usually need a special mounting. A telescope mounting must not only support the telescope's weight no matter where in the sky it points, but also must not vibrate. The telescope's magnification will amplify vibrations, which makes for jittery, useless images. A good mounting often weighs much more than the telescope it supports.