Orion 8548 Explorer II 7-21mm Zoom Telescope Eyepiece The Orion Explorer II Zoom eyepiece lets you change magnification power without physically changing eyepieces A twist of the zoom eyepiece housing changes focal length from 21mm to 7mm, providing a variety of viewing magnification powers Eyepiece consists of four elements which are fully coated with anti-reflection coatings for clear, contrasty views Exceptionally long 33mm-to-16mm eye relief range allows eyeglass wearers to enjoy the entire view while still wearing their corrective lenses without vignetting The 1.25" barrel is threaded for use with like-sized Orion eyepiece filters