Continuing the theme from the last issue, this article considers the use of hands-free magnification devices that are suitable for a wide range of tasks for both low vision patients and hobbyist.
When considering magnifiers, the term ‘hands-free’ means different things to different people; for many, it describes the ability to have full use of both hands to hold a book or a paper, rather than holding a device and moving it across the page. The objective of this second article is to focus upon simple, readily available solutions that can be dispensed to both visually impaired and normal sighted individuals that leave the full use of both hands to perform a specific task, or to adopt a more natural reading posture.
Clamps and stands
One of the simplest methods to create a hands-free solution is to clamp a standard magnifier into a flexible or adjustable stand so that the hands may hold the object beneath it (see Figure 1: A large diameter hand magnifier has been clamped into a long, flexible double-ended clamp with the other end of the clamp firmly attached to the table). Most handled magnifiers can be securely slipped into a sturdy double-ended clamp and moved into position for the task. When choosing a stand for this purpose, it is important that the clamps at either end are strong, preferably with additional rubber pads to secure the magnifier without damaging it and that the flexible part of the stand is both bendable to place into position but sturdy enough to take the weight of the device.
Desktop illuminated magnifiers offer the same flexibility as a double-ended clamp but with the advantage of additional lighting (see Figure 2: Table clamp magnifiers offer hands-free magnification afforded by the long, flexible stand length. This model offers 2.5x magnification with the advantage of additional LED task lighting). Offering low to medium levels of magnification with a long flexible stand, these relatively large diameter lenses may either be clamped securely onto the desk or table or come as free-standing units with a heavy base to offer stability. The user may wear either their distance or near spectacle correction and the object is moved into the right point beneath the magnifier; this type of magnifier is also used frequently by a wide variety of professionals such as beauticians, dentists and chiropodists.
Detachable or integrated pull out stands for hand held magnifiers offer the user a quick, hands-free solution with the flexibility of a portable device (see Figure 3: The hand magnifier is slipped securely into a sturdy stand, converting it into a stand magnifier in one position and a hands-free option in another). Being able to convert a hand magnifier in this way offers a low cost option, which may be attractive where personal or hospital budgets are a priority.
However, it is important to check that the stand is sturdy and simple enough for a visually impaired patient to manage, especially if being used on a daily basis. Some stand designs may appear innovative, but may be flimsy or have a weak connection to the body of the magnifier.
Suspended magnifiers (also termed neck or chest magnifiers) are typically low powered and lightweight and are suspended around the patient’s neck by a simple cord, with feet resting on the user’s chest (see Figure 4: The patient is using a 2x suspended magnifier. The feet have rubberised or foam pads that stop the magnifier from slipping over the user's clothing and the transparent frame reduces unwanted shadows allowing more light to reach the task). These types of magnifier are used by both the visually impaired and hobbyists who seek a hands-free solution for common tasks such as knitting or crafts. As the lens diameter is quite large, lens powers tend to be low although many designs will have a higher-powered ‘insert’ of a small diameter to assist with detail when required. Similar to the flexible clamp solution, the patient may wear either their distance or near spectacle correction and then adjust the position of the hands beneath the magnifier to find the appropriate focusing distance.
Often patients are keen to explore the possibilities of ‘stronger glasses.’ However, the challenge that the practitioner faces is to demonstrate and persuade the patient that the more powerful the positive lens in the trial frame, the shorter the working distance will be. Spectacle magnification in its simplest form refers to magnification achieved by incorporating a lens in the spectacle plane. With optical magnification of this nature, an emmetropic patient will be working at or slightly within the focal length of the positive lens magnifier. For example, for a 4x (+ 16D) spectacle magnifier, the working distance will be approximately 6.25cm. For a myope who replaces his glasses with the same spectacle magnifier, the working distance will reduce further, compensating for the spherical component of his myopia. Conversely, if the patient is hyperopic, the object will be held further than the anterior focal length of the positive lens as a proportion of the lens power will be ‘used up’ correcting the patient’s distance refractive error.
There is a variety of dispensing solutions available from a number of prescription houses and low vision suppliers. Ranging from approximately 1x to 12x, spectacle magnifiers are available in a diverse range of single lens forms including multi-order diffractive (MOD) lenses, biconvex aspheric hyperocular lenses and high-addition bifocal segments to name but a few. Where higher levels of magnification are required, a combination of lenses may be housed in a carrier mounted onto the patient’s spectacle lens creating levels of magnification as high as 20x or more.
For patients who prefer to remain binocular, base in prism or decentration will be required to avoid diplopia at shorter working distances; most will require base in assistance when holding material consistently closer than approximately 25cm. There are no hard and fast rules for how much prism or decentration is required for specific working distances as individual interpupillary distances vary. Therefore, if an individual order is to be made, then practitioners are advised to check the amount of decentration or base-in prism required for each patient. That said, there are a number of successful ‘ready-made’ spectacle ranges available that incorporate base-in prism to assist convergence. However, the amount of prism calculated is based upon the amount of positive power in the spectacles and, therefore, assumes that the patient is at or close to emmetropia. These ready-made ‘prismatic spectacles,’ as they are often called, are available from several manufacturers with powers ranging from approximately +4.00D to +14.00D with prism incorporation of approximately 4 Δ to 14Δ base in for each eye, respectively (with slight variation in values between manufacturers). However, as most acquired conditions affect each eye unequally it is more usual to prescribe magnification to the better eye, and either use a balancing, frosted or opaque lens in the fellow eye to improve visual comfort (see Figure 5: The patient is using a half eye frame into which a high-powered multi-order diffractive (MOD) lens is providing 3x magnification to one eye. The fellow eye is frosted. Both lenses are symmetrical and interchangeable so that the spectacle magnifier can be made up during the consultation from a bank of lens powers. The patient gains additional working distance by placing the frame further down her nose).
When considering high additions, patients will still require astigmatic errors over 1.50-2.00DC to be fully corrected. In such cases, the above examples of ready-made spherical spectacle magnifiers may not be the most appropriate choice. Any additional plus power can be incorporated in one of two ways: either the combined high addition is added to the patient’s distance prescription and ordered as a standard lens through the prescription house in the usual way; or the additional power may be added onto the front surface of the patient’s spectacle lens within a carrier or as a clip-on.
Positive lens attachments are available in low and mid range powers and may be attached by a simple clip-on. Such solutions are relatively inexpensive and offer flexibility in power change if the vision alters. Binocular attachments are available in relatively low powers to approximately 3x; single lens clip attachments are available in higher powers to approximately 7x where binocularity is not a priority (see Figure 6: The patient views a smartphone by placing it at the anterior focal length of the binocular 1.7x clip on lens. As the lens attachment is mounted on the end of a stalk, the patient has the advantage of extending the overall working distance by a few more centimeters).
Alternative solutions to clip on lenses include bifocal positive power lens ‘buttons’, which may be attached to the front surface of the patient’s current spectacles in several ways. One method is to cement a carrier onto the front surface of a patient’s prescription into which interchangeable buttons may be secured; this method allows for a flexible system and is useful if the patient’s VA drops over time. Another method is to provide a self-adhesive lens button that can be mounted directly onto the lens surface (see Figure 7: Self-adhesive bifocal 'buttons' of only 1g in additional weight may be attached within moments by simply removing a peel-off backing; this negligible additional weight adds hands free magnification of up to 10x to the patient's existing spectacles). Both solutions offer magnification up to 10-12x.
Simple spectacle mounted telescopic units
For patients who seek a spectacle-mounted option but struggle with shorter working distances, spectacle-mounted telescopic magnification offers an increased working distance compared to a spectacle magnifier of the same power.
Amongst the simplest of designs are the self-adjusting spectacle magnifiers providing 2-3x magnification and which are based upon a simple Galilean telescopic design, with a negative eyepiece and positive objective lens. Relatively low in cost, some designs are able to compensate for +/- 3D of spherical ametropia by using the cog on the side of the spectacle frame. Several designs currently exist in the marketplace and it is worth taking time to review both the quality of the lenses used in the magnifier and the mechanics of the lens adjustment system. For the latter, a robust mechanism is desired when prescribing to visually impaired users who may have less manual dexterity when adjusting the lenses. Furthermore, if a patient is to remain binocular then it is preferable to be able to adjust each eye individually rather than have a single adjustment that affects both eyes simultaneously. This type of spectacle telescope magnifier has become popular for both visually impaired users and hobbyists, as they are low in weight, cost-effective and relatively simple to use and adjust.
Where astigmatic correction is required, clip on versions have more recently become available for both distance and near (see Figure 8: This lightweight clip-on telescope adds only 23g to the overall weight of the spectacles. With 2x magnification, the unit's 35cm working distance is significantly extended compared to a spectacle magnifier of the same magnification with a working distance of 12.5cm). This type of system will clip onto the majority of spectacle frames and has a wide PD tolerance. The height of the system is fully adjustable so that the centre of the eyepiece lenses can be adjusted to coincide with those of the user’s own spectacle lenses. Weight is further reduced, as an integral focusing system is no longer required. The telescope may also be flipped up when not in use. By using a design based upon a Galilean telescope, there is no requirement for incorporating prisms that would be necessary in more complex Keplerian designs.
Spectacle mounted telescope solutions in higher powers are available for both distance and near. In the past, telescopic units were cemented onto the front surface of the patient’s spectacles or screwed into a glazed spectacle mount. More recently, however, self-adhesive mounts have become available that have significantly reduced glazing costs (see Figure 9: A telescopic base unit of 2.5x distance magnification can be screwed into the self-adhesive mount attached to the front surface of the patient's spectacle lens. Additional reading caps convert the distance unit into a hands-free magnifier for near with a magnification range up to 10x). Units may now be mounted onto the patient’s spectacles during the appointment making previously assumed ‘complex’ telescopic magnification an increasingly viable and cost–effective option for both practitioner and patient.
With an ageing population, magnification solutions are becoming a fundamental presence within optical practice. More practitioners are seeking to invest in devices that offer enhancement to standard spectacle correction for specific tasks and hobbies and, therefore, previously termed ‘low vision’ aids are becoming more popular for those with near-normal vision. With improvements in materials, lens technology and with new innovative and attractive designs, the range of products has become extensive in recent years. Patients seek good quality devices that are robust, simple and practical to use and there is now a wealth of products in the marketplace to meet these needs.
About the author
Jane Macnaughton is currently an associate trainer for Associated Optical Ltd, specialist optometrist at the Leicester Royal Infirmary, and runs a private low vision practice at the Nuffield Health Leicester Hospital. She has lectured widely in the practice of low vision from undergraduate to post graduate level and is the author of Eye Essentials: Low Vision Assessment (2005).
- Dickinson, C (1998) Low Vision Principles & Practice. Butterworth Heinemann
- Jackson, J, Wolffsohn, JS. Low Vision Manual (2007) Butterworth Heinemann
- Macnaughton, J (2005) Eye Essentials Low Vision Assessment. Elsevier Butterworth Heinemann. Reprinted by kind permission, ABDO (2009)