Apparent image quality of magnifiers depends on amplitude of accommodation

Abstract

We investigated the importance of the role of amplitude of accommodation in assessing the usefulness of magnifiers. Accommodation must be exerted to scan across the image produced by a magnifier that exhibits astigmatism and curvature of field. In general, increasing amounts of negative accommodation are required to scan from the center to the edge of the image field of view (FOV) when a simple magnifier is set for nominal magnification. It is necessary to refocus the magnifier (move it closer to the object plane) to convert the demand to positive accomodation. We used the OSLO program to design and evaluate the oblique astigmatism of four common types of magnifiers: Equiconvex, Best-form, Aspheric, and the Steinheil triplet, in powers of 10, 20, 40, 60, and 80 D. We found that the circle of least confusion (CLC), produced by the Equiconvex and Best-form lenses set at nominal magnification, lie on a surface that demands as much as 2.75 D of negative accommodation to view the CLC at the edge of a 30 degrees FOV. By refocusing the magnifier, the CLC at the edge of the field can be imaged at infinity, i.e., have zero vergence, and the demand for negative accommodation may be eliminated, but then about 2.4 D of positive accommodation is required to scan to the center of the field. The Aspheric and Steinheil magnifiers largely corrected the astigmatic errors and flattened the field. Less than 0.75 D of accommodation was sufficient to scan from the edge to the center of the object. Depending on the degree of uncorrected astigmatism and field curvature, presbyopic patients, lacking accommodative ability, will perceive constricted in-focus zones in the image field. They will continuously have to refocus the magnifier to bring adjacent zones into best-focus. This condition may cause a presbyope to reject a magnifier thought to be of good quality by a young clinician who can accommodate for best-focus across the entire FOV.