Magnifiers and Other Optical Devices

The magnification effects produced by a piece of convex glass when held above written text have been known for many centuries. Indeed the first spectacles were basically a pair of such magnifiers mounted into a frame. Consequently magnifiers, and magnifying spectacles, have been used as vision enhancing devices for countless years. The basic principal underlying the use of any magnifying device is to provide an increased image size that will cover a larger area of the retina. This increased image coverage of the light sensitive retina, compared to the unmagnified image, allows the brain to interpret the image more easily.

Optical magnifying devices fall into two distinct categories, simple magnifiers and telescopic lens systems. Of these the former are probably the easiest to acquire and use. Telescopic devices tend to be a more specialised unit, normally available through an eye care professional, and require more skill in their use. Magnifying devices are available in a large range of powers and forms and are a very effective aid to vision.

However, due to physical limitations of optics, there are finite values for magnification, area and field of view. As a general rule of thumb, when considering any magnifier, it should be remembered that the larger a magnifier the weaker its magnification. Aberrations arising from the optics of the lenses will cause effects such as distortion and colour fringes. Consequently magnifier designers have to balance aberrations against benefits when producing their magnifiers.

Simple magnifiers: usually consisting of a high power positive lens mounted in some form of handle or stand, magnifiers are available in magnifications from 1.5x through to 20x. They are available in non-illuminated, illuminated, stand, hand held, spectacle mounted and hands free mountings. Up to 4x magnification the simple magnifier is a relatively easy device to use, being tolerant of working distance and usually of a relatively large area.

Once the magnification begins to exceed 4x the use of the magnifier becomes more precise. The main problems experienced when trying to adapt to using a stronger powered magnifier are principally related to poor understanding of the technique and limitations of the unit. When using magnifiers of 4x power or above, it is important to forget the ‘Sherlock Holmes’ approach to using a magnifier. Strong magnifiers need to be held quite close to the subject matter, the actual distance being dependent upon the lens power.

This distance is typically in the range of 15mm to 45 mm. Similarly the distance from the magnifier to the user's eye is critical to ensuring optimum magnification and field of view.

Again the actual distance is dependent on the magnifier power but typical examples are an eye to lens distance of 150mm for a 4x unit reducing to 40mm for a 10x unit. It must also be emphasised that the magnifier will magnify spaces as well as text so the actual reading process through a magnifier will be slower than previously experienced with ‘normal’ vision.

It is also common for users to report difficulty in following lines of text due to the reduced area of the unit and it is advisable to use some form of line guide, such as a typograph, to assist. To assist with reading lines of text a number of manufacturers have produced magnifiers with built in line guides.

These take the form of simple bar magnifiers with engraved red line guides, dome magnifiers with part frosted bases (so called Visual Tracking Magnifiers), magnifiers with additional bar magnifiers built into the handles and carefully designed lighting systems that produce a bright line of light within the magnifier's field of view.

Magnifiers will also magnify any slight hand tremor or movement and it is important that the units are supported firmly when being used. The use of a rigid table or clipboard is essential as the magnification increases. One final family of magnifiers use a theory that results in a very thin sheet magnifier.

By moulding concentric prismatic ridges on the surface of a sheet of plastic, Fresnel magnifiers can be made to enable areas as large as A4 sheets. However there is a penalty to pay fresnel magnifiers are of a relatively low power and there is some image degradation caused by the design. A similar theory is used for the TV screen magnifier.

Telescopic Magnifying Devices
Using optical telescope principles a number of manufacturers have produced spectacle mounted telescopic reading devices. Foremost in this field would be the Keeler range of LVA telescopes that utilise a unique mounting ring that attaches onto a spectacle lens. Once attached it is possible to screw various telescopes onto the spectacles for near vision and far vision use. The range of telescopic units available includes wide area reading units, read-write units, short focus distance units and rapid focus distance unit.

The advantages of these units are hands free operation, reasonably good field of view, excellent optical performance and ease of use. Like their simple magnifier counterparts there are a few important points to remember when using these devices if they are going to be beneficial. Firstly the working distance - all of the near vision units within this category require a change in attitude towards reading.

The distance from the spectacle mounted unit to the reading matter needs to be quite short, typically 150mm or less (depending on power/type) and as a result the user must learn to hold print closer. Secondly the demands upon the muscles around the eyes are increased with the extra concentration required when using any high power device. Thus it is advisable to use the units for short periods of time only and to have frequent rests whilst using.

Ultimately the choice of which low vision device is most appropriate to the requirements of an individual can only be assessed by a trial of all the available units. As a rule the least practical devices are those that are the most requested - spectacles. Generally the simpler the unit the more easily it will be to use.

In the UK low vision services are provided by a number of agencies, ranging from vision charities, high street Opticians, specialist Low Vision Practitioners and also under the auspices of the Social Services and the Hospital Eye Service. Some of these agencies are listed on our link page, and to ensure the optimum device is made available, a full Low Vision Assessment is always advisable.

The benefits of the various types of available magnifier units are:

Type of Magnifier		Advantages		Disadvantages
Hand Held Device		• Inexpensive • Easily acquired • Convenient • Tolerant of poor technique • Internal illumination (if required)		• Difficult to hold • Reduced field of view at longer ranges • Unusable if Hand Tremor Present
Stand Mounted Device		• Hands free use • Tolerant of tremors, etc. • Internal illumination (if required)		• Require flat surface • Stand may obstruct light • Stand may obstruct use
Spectacle Mounted		• Hands free use • Wide field of view • More acceptable to spectacle wearers • Easily upgraded • Modular designs		• Very short working distances • Very blurred distance vision • Illumination difficult • Binocular units • Restricted to 3x magnification • Initially expensive