Now even the casual telescope user can enjoy the benefits of zoom eyepiece technology. A twist of the affordable Explorer II Zoom housing changes the focal length from 7mm to 21mm, or to any focal length in between. That makes it ideal for locating an object at low magnification, then zooming in at higher magnifications for closer inspection. Without changing eyepieces! Actual magnification range will depend on the focal length of your telescope. It works equally well for astronomical or terrestrial viewing. This four-lens eyepiece boasts fully coated optics and exceptionally long eye relief ranging from 16mm to 33mm. 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. A nice value in a quality zoom eyepiece!