Experimental Study of Refraction Effects of Nominally Plano Ophthalmic Prisms and Magnifying Lenses

Nominally plano ophthalmic prisms give autorefraction results similar to those predicted on the basis of how effective powers change with pantoscopic tilt, and magnifying lenses give autorefraction results similar to those predicted on the basis of vergence changes. Without appreciation of the optics involved, these effects might wrongly be considered artifacts.

The purpose of this study was to investigate the interactions of autorefractors with lenses and prisms.

There were 15 adult participants across three experiments, with a range of ages and refractions. In experiments 1 and 2, participants wore frames containing base-up and base-down nominally plano prisms. In experiment 3, participants wore a lens that produced either 6.3% magnification or 5.9% minification, depending on which surface faced the eye. Autorefracting instruments with different operating principles were used: Shin-Nippon SRW5000 autorefractor, Grand Seiko 5100K autorefractor, Hoya AR-530 autorefractor, a Complete Ophthalmic Analysis System-High Definition wavefront sensor, and Tomey FC-800 autorefractor. A theory on the likely effects of magnifying lenses was presented.

For ophthalmic prisms, refractions showed results similar to those predicted on the basis of how effective prism powers change with pantoscopic tilt. As tilt increased, base-up prism gave more positive mean refractions and more negative horizontal/vertical astigmatism and vice versa for base-down prisms. In the presence of 10° tilt, 8Δ base-up prisms and 8Δ base-down prisms had different effects by a mean of 0.36 diopters. Magnifying lenses affected refractions similar to those predicted on the basis of vergence changes, with 6% magnification and minification producing mean changes of -11 and +8%, respectively, in absolute mean refraction. There was no strong evidence that different instruments had different effects.

The results have implications for studies in which prisms and lenses are placed in the front eyes, such as accommodation studies using thick lenses close to the eyes to stimulate accommodation rather than by changing object distance.