The Explore Scientific 3X Focal Extender (2.0-inch O.D.), accepts both 1.25-inch O.D. and 2.0-inch O.D. format eyepieces and camera adapters. Fully EMD-Coated on all air-to-air lens surfaces, with all lens elements edge-blackened to produce superior contrast, this focal extender adds some real versatility to your telescope accessory line-up. First of all, it allows you to increase the magnification of any eyepiece with the same barrel size that you already own. In other words, 3 eyepieces offer 6 magnification choices with the addition of a focal extender. Secondly, most of the time, lower magnification eyepieces such as the 30mm 82 Degree Series Waterproof Eyepiece have better eye relief than higher power models. When using a focal extender to increase your magnification, you get to enjoy the generous eye relief of the lower power eyepiece, and that's quite a bonus, especially for eyeglass wearers or for those with fast telescopes who can really push high magnification eyepieces. Remember, we also make 1.25-inch O.D. Focal Extenders in 2X, 3X and 5x models, so there is something for every style of telescope.