TeleVue Nagler eyepieces are the premium eyepieces that started the revolution in ultra-wide field eyepiece design.
They have six to seven lens elements that give up to three times the field area of a Plössl eyepiece. While not significantly sharper than a Plössl in the center of the field, they have a much wider field and are sharper overall from edge to edge. They are also up to ten times sharper at the edges of the field than older wide angle designs such as Erfles. Their field is so wide that you have to move your head from side to side to see all of it, making looking through a Nagler more like looking out a picture window into space rather than looking through an eyepiece.
All have superb color correction, with little ghosting or field curvature. Their very wide fields make them convenient for planetary and globular cluster observing with Dobsonians, as the wide field makes it easier to locate the objects and you have much longer to observe before the object drifts out of the field and the scope has to be repositioned.
They are designed for state-of-the-art observing with scopes having focal ratios down to f/4. They are marvelous for open clusters, nebulas, and globular clusters with any scope, but are particularly effective with fast focal ratio reflectors.