Aplanatic

Ap·la·nat·ic

Having two or more parts of different curvatures, so combined as to remove spherical aberration; -- said of a lens. (Opt.)

Aplanatic focus: the point or focus from which rays diverging pass the lens without spherical aberration. In certain forms of lenses there are two such foci; and it is by taking advantage of this fact that the best aplanatic object glasses of microscopes are constructed.