Myopia control and modulation transfer functions for multisegment spectacle lenses

Abstract

Introduction: Multisegment (MS) spectacle lenses, providing a distance correction with a clear central area and by having an array of small, positively powered lenslets in the periphery of the front surface, have proved effective in slowing childhood myopia progression. Debate continues as to whether their mechanism of action is due to through-focus effects or to the image contrast changes due to the inclusion of the lenslets. This study explores the second possibility by modelling the performance of a combined MS lens-eye optical system in terms of its modulation transfer function (MTF) under various conditions. Methods: The optical design program Ansys Zemax OpticsStudio was used to determine distance MTFs for the combination of either a single-vision or a Hoya MiyoSmart MS lens with a 4 D myopic eye model. Conditions included axial and peripheral objects with co-axial lens and eye, and rotating the eye away from the lens axis to observe objects through the lenslet-covered region of the lens. Visual resolution under each condition was estimated. Results: Observing objects through the lenslet array lowered modulation transfer in comparison with that given by the single-vision lens, especially as spatial frequency increased. In peripheral observation at a field angle of approximately 32.5 degrees, imagery was poor. Foveal image quality was better with axial viewing through the clear MS lens centre than when the eye was rotated by approximately 30 degrees. Conclusions: Optimal visual resolution during MS lens wear is achieved when fixating through the clear, central area of the lens. Under these circumstances, objects at 20–50 degrees from fixation are seen through the lenslet-covered region of the carrier which produces a low-pass spatial frequency filtering effect. Here, visual resolution is limited to a few cycles per degree so that any growth control mechanism must rely on low spatial frequency information.