Aspheric surfaces provide a very necessary increase in the number of degrees of freedom needed to design wide-field eyepieces with large eye relief distance to focal length ratios. New economical diamond-turning lathes such as the Optoform 50 give a new flexibility to prototype manufacture with plastic materials which extends into medium-sized production quantities. This paper will explore the free use of aspherical surfaces in wide-field visual systems. The aspheric is now the general case and not a small modification of a spherical surface. The computing procedures used must provide the best balance between freedom and restraint.
Michael H. Freeman,
"Wide-field aspheric optics for visual applications", Proc. SPIE 2774, Design and Engineering of Optical Systems, (23 August 1996); doi: 10.1117/12.246693; https://doi.org/10.1117/12.246693