An eyepiece is a type of lens that is attached to a variety
of optical devices such as telescopes and microscopes. It is called an
eyepiece because it is usually the lens that is closest to the eye when
someone looks through the device. The objective lens or mirror collects
light and brings it to focus creating an image. The eyepiece is placed
at the focal point of the objective to magnify this image. The amount
of magnification depends on the focal length of the eyepiece.
An eyepiece consists of several lens elements in a housing with a barrel
on the bottom. The barrel is shaped to fit in the opening on a telescope.
The image can be focussed by moving the eyepiece nearer and further
from the objective, and most instruments have a focusing mechanism to
allow this without requiring that the eyepiece be manipulated directly.
The eyepieces of binoculars are usually fixed in place, causing binoculars
to have a pre-determined magnification and field of view. With telescopes
and microscopes, however, eyepieces are usually interchangeable. By
switching the eyepiece, 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 eye relief for the person who looks through them.
Modern research-grade telescopes do not use eyepieces. Instead, they
have high-quality digital cameras mounted at the focal point, and the
images are viewed on a computer screen. Some amateur astronomers use
their telescopes the same way, but most still look through eyepieces.