ABSTRACT
An eyepiece is a lens system with an external entrance pupil. The eye should be placed at this entrance pupil location. The distance from the entrance pupil to the first lens surface is called eye relief. Eye relief should be at least 10 mm (to obtain adequate clearance for eyelashes), 15 mm to provide more comfortable viewing, and 20 mm for people wearing spectacles. In the case of eyepieces used for rifle sights, the eye relief should be at least 3 in. to allow for rifle recoil (see Figure 14.4 and Figure 35.4). For an example of a zoom eyepiece, see Figure 35.11 that shows an eyepiece that zooms from 40 to 20-mm effective focal length.
