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THIS ARTICLE IS BEST READ ON A PDF.
Last week, we looked at some of the factors to be considered when prescribing hand magnifiers, with particular regard to their use in conjunction with spectacles. This week we will consider a few points relating to stand magnifiers.
Which spectacles with a stand magnifier?
Stand magnifiers are obviously designed such that the lens is maintained at a constant distance from the object to be viewed, such as a page of a newspaper. Were this distance to be the actual focal length of the lens then the light leaving the appliance towards the viewer's eye would be parallel. As such, the image would be clear for an emmetrope or someone corrected for distance. It would also be the case that the field of view could be increased by moving toward the aid without any impact upon the actual magnification of the image.
This scenario is hardly ever found, however, as stand magnifiers are produced such that the lens is set at a distance somewhat less than the focal length of the lens. This means that the effective magnification is less than that possible for any particular lens and also that the emergent light is divergent. This requires the use of accommodation, or a reading addition to see the image clearly.
This apparent anomaly in design (requiring a change of glasses in many cases and a reduction in possible available magnification) is explained by an improvement in the quality of the image. As the lens approaches its focal length from an object, the quality of the image degrades with a significant increase in aberrations.
With poorer-quality lenses, colour fringes as a result of chromatic aberration (blue on one side of details and yellow on the other), become very apparent. By setting the lens closer than the focal length a much sharper image is obtained which, when images are magnified, improves the visual performance of the viewer.
This also has the effect that the vergence of light meeting the viewer's eye will increase as the eye approaches the aid. Many patients, frustrated with the limited field of view through a strong stand magnifier, move closer towards it to see more of the words on a piece of printed text.
This means that the image will become blurred unless the viewer is able to accommodate the extra amount to compensate for the vergence or if they lift the aid up from the page to reduce the emergent vergence (and thereby removing any of the benefit of the stand magnifier in the first place).
Many stand magnifiers, such as the Eschenbach range of illuminated aids shown in Figure 2, include several figures on the casing. As well as an indication of the dioptric value of the lens and the magnification value, there is also a third number such as 250 or 300. This is the recommended working distance in millimetres at which the stand magnifier should be used when viewed through a 'standard' reading addition. This addition is likely to be a +2.50DS add, but manufacturer literature should specify this.
The actual value of the accommodation or addition required may be worked out from basic principles. Take the example of a +6.00DS lens in a stand magnifier which is set at 10cm from the page. Light hitting the lens from the page will have the vergence of -6.00D, such that the emergent vergence is -4.00.
The apparent image will be seen to be at -25cm relative to the lens. If the eye is held right up to the stand magnifier then +4.00 addition or accommodation is required. For a lens to eye separation of 5cm, then the addition needed will be 100/(25+5) or 3.30D. For a separation of 25cm, more realistic for most users, the addition or accommodation will be 100/(25+25) or 2.00DS.