There are different kinds of eyepieces, depending on the diameter of the barrel:
- Telescope eyepieces: They have a barrel diameter of 1.25 inch or 0.95 inch
- Stereo microscope eyepieces: They have a diameter of 30mm or 26mm
- Compound microscope eyepieces: They have a diameter of 23mm
Eyepieces also differ in their eye-relief (eye point). This is how far away the eye can be from the front lens of the eyepiece. Different eyepieces also have a different field of view.