15mm focus length 15mm is a intermediate eyepiece fit for observing the lunar surface planets binaries and bright nebula interior. 4 Lenses in 3 Groups: Multiple group lens combination to improve optical performance; the eyepieces delivers sharp images with good color correction and contrast. 16mm eye relief: Generous 16mm eye relief allows comfortable viewing distance and lets eyeglass wearers see the entire field of view. Full multi coated optical and 4-elements in 3 groups allow more light pass through to the eyepieces thus providing a brighter sharper and higher contrast images Rubber eye cup You don't have to train yourself to place your eyebrow or cheekbone in a spot to hold onto the correct small spot that gives you the full of view.Perfect for eyeglasses wearers and those people have long eyelashes Ultra Wide Angle and Crisp Image ---66 degree wide angle eyepiece; ultra wide angle provides a widely field fit for observing the lunar planets binaries, the moon; star clusters and the Milky Way, etc. ---FMC film and 5 elements in 3 groups maximize the light transmission and minimizes internal reflection and maximizes contrast, provide crisp images for you.