Tolerance to tolerances: some thoughts on the 1979 American National Standard recommendations for prescription ophthalmic lenses.

The 1979 revision of the ANSI Z80.1 Ophthalmic Lens Standards resulted in rather significant changes affecting three areas of ophthalmic lens requirements: 1) central refractive characteristics, 2) prismatic characterstics, and 3) factors relating to the magnification properties of lenses. Based primarily on what an optical laboratory could consistently produce, the ANSI Ophthalmic Lens Sub-committee decided to widen some of the tolerances over those expressed in previous revisions. This paper attempts to show how these new tolerances relate to certain aspects of patient sensitivity to errors induced by ophthalmic lenses conforming with the new revision.