Eyepiece
An eyepiece is a type of lens that is attached to a variety of optical devices such as telescopes and binoculars. It is called an eyepiece because it is usually the lens that is closest to the eye when someone looks through the device.
The eyepieces of binoculars are usually fixed in place, causing binoculars to have a pre-determined magnification and field of view. With telescopes, however, the eyepiece is often interchangable. By switching the eyepiece that is attached to the telescope, the user can adjust what is viewed. For instance, eyepieces will often be interchanged to increase or decrease the magnification of a telescope. Eyepieces also offer varying fields of view, and differing degrees of focal length of the eyepiece, when combined with the focal length of a particular telescope, is what determines the magnification that will be achieved. For instance, an astronomer using a telescope with a focal length of 1 metre (1000 mm), could obtain a magnification of 40 times by using an eyepiece with a focal length of 25 mm. The astronomer could increase this magnification of the image to 80 times by exchanging it for a 12.5 mm eyepiece.
The focal length is probably most frequent of properties referred to by amateur astronomers when considering eyepieces. It is usually expressed using millimetres, although some amateur astronomers prefer to describe eyepieces by the magnification that they provide when used with their particular telescope.
Apparent field of view
This property describes the field of view that the eyepiece will appear to have when not connected to any instrument. The actual field of view will depend on the magnification that applies when the eyepiece is connected to a particular instrument.
Barrel diameter
This describes the diameter of the slot in the telescope that the eyepiece must fit into. The diameter is most commonly expressed using inches. Two common barrel diameters are 1.25" and 2".
