A twist of the zoom eyepiece housing changes focal length from 21mm to 7mm, providing a variety of viewing magnification powers. Excellent contrast and sharpness - making the zoom also of interest for planetary observing. | Exceptionally long 16mm-to-33mm eye relief range allows eyeglass wearers to enjoy the entire view while still wearing their corrective lenses without vignetting. The Zoom eyepiece lets you change magnification power without physically changing eyepieces. | Apparent field of view is 30- degree at 21mm and 43- degree at 7mm. Rubber traction ring ensures a good grip. The 1.25" barrel is threaded for filters. Actual magnification range will depend on the focal length of your telescope. It works equally well for astronomical or terrestrial viewing. | Eyepiece consists of four elements which are fully coated with anti-reflection coatings for clear, high contrast views. The particularly large eye lens coupled with the long eye relief means the Zoom provides particularly good viewing comfort, the eyecup can folded down - handy for spectacle wearers. | Comfortable viewing at every focal length setting - superior to most fixed focal length eyepieces, especially at the shorter focal length settings, precise and stepless adjustment from 7mm to 21mm. A nice value in a quality zoom eyepiece!