Stand magnifiers for low vision: description, prescription, assessment

Abstract

Stand magnifiers are still one of the most commonly prescribed classes of low vision devices. Their performance can be difficult to understand because stand magnifiers usually do not give an image at infinity. This review summarises the methods of describing image enlargement for stand magnifiers, emphasising their relationship to equivalent viewing distance (EVD). This is done in terms of the underlying optical equations, and measurement methods, and methods of prescribing. In the past, methods of determining EVD have been somewhat indirect, requiring accurate measurement of lens power, and image position. The use of digital photography provides an alternative, more direct, simpler method of determining EVD, which can be accomplished in‐office. This method is described and it is demonstrated how it gives comparable results to older methods with small, clinically non‐meaningful differences, that may be due to differences in image distance reference planes. Describing the performance of stand magnifiers in terms of their dioptric power, or in terms of ‘nominal magnification’ or ‘trade magnification’, is imprecise and misleading. It is better to use indices such as equivalent viewing power and EVD, which take into account the magnifier dioptric power, the image position of the magnifier and the distance a patient is from the magnifier. While EVD is a useful index for prescribing stand magnifiers, manufacturers do not always provide sufficient technical details to determine EVD for their stand magnifiers, and available tables of EVDs are more than a decade old and are likely to need updating. Photographic comparison provides a method for determining EVD, and this method can also be applied to other low vision devices.

2022 Glenn A. Fry Award lecture: Enhancing clinical assessment for improved ophthalmic management

“…Visual impairment from ocular disease is generally uncorrectable, so low-vision rehabilitation focuses on making the best use of the remaining vision [102][103][104][105] such as through the use of colored lenses, [106][107][108] optical magnifiers 109,110 (including optimized page illumination), 111 and electronic vision enhancement systems, 1 1 2 -1 1 4 which we and other research groups have evaluated. We developed the terminology "electronic vision enhancement systems" to better describe the use of technology that has emerged in instrumentation for those with low vision than the traditionally used "closed-circuit television" terminology.…”

Possession of Low Vision Reading Devices Among Residents Living in a Retirement Facility

“…Low vision devices are also available through non-optical means (such as supplemental lighting), enlarged print materials and enlarged numbered devices, and assistive technology (such as text-to-speech devices). Among older adults with visual impairments, optical magnifiers have been reported to be one of the most frequently used devices to aid reading (Carkeet, 2019;Gobeille et al, 2018;Lew et al, 2015). Overall, the use of low vision devices to compensate for visual impairment is a simple and effective means to reduce degree of disability, improve vision-specific quality of life (Heravian et al, 2016) and reading ability (Choi et al, 2018;Patodia et al, 2017), though evidence for the use of vision-specific assistive technology such as optical magnifiers to improve reading performance of daily tasks is low (Smallfield et al, 2013;Smallfield & Kaldenberg, 2020) Ownership of prescribed low vision devices, as well as factors associated with their use, has been examined in older adults with low vision (Lorenzini & Wittich, 2020).…”

Looking and seeing beyond 2020

“…In contrast to the Kryptok of 1919, Jalie (a name synonymous with ophthalmic optics) reviews modern spectacle lens designs including free‐form manufacturing techniques to minimise the visual impact of spectacle lens aberrations. Carkeet also examines the optics of stand magnifiers, a commonly prescribed low vision aid despite significant advances in electronic devices in recent years . A novel method utilising digital photography to determine the equivalent viewing distance of stand magnifiers is also described.…”