To correct myopia, a minus lens can be added in front of the eye, so the eye can focus at distance. Hence, one of the methods of correcting myopia is using negative magnification lenses with differing degrees of magnification according to the severity of the condition.
Hypermetropia or hyperopia, is a common refractive error in children and adults, according to Dart D.(2010). The effects and the symptoms on a subject varies according to how high is the hyperopia, the age of the subject, the status of the accommodative and convergence system and the demands placed on the visual system. Individuals with uncorrected hyperopia may experience symptoms like: blurred vision, headaches and eyestrain, while reading, accommodative/binocular dysfunction.
Plus lenses, used to correct long –sightedness, have a side effect of making the images on the back of the eye (retina image) appear larger than they should and minus lenses for correcting short – sighted conditions, make the retina image smaller.
As well as with the minus lenses, one of the difficulties that might be found in a plus power lens is the thickness of it. The larger the lens, the greater is the thickness and the weight of it. But in this case the problem is within the centre of the lens rather the edge of it. From the cosmetic point of view, plus lenses become more bulbous as they increase in power.
Presbyopia is, according to A Glasser (2010), the gradual and progressive age-related loss of accommodative amplitude. The progressive loss of accommodation begins early in life and culminates in a complete loss of accommodation by about 55 years of age.
Presbyopia ‘s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the lens’s curvature from continual growth and loss of power of the muscles that bend and straighten the lens(known as the ciliary muscles) have also been postulated as its cause.
As a dispensing optician, a special attention needs to be paid when the subject is collecting the glasses to ensure that all the relevant information related to the lenses and the frames are passed to the subject. As mentioned within the text, the minus powers have the ability to make the object appear smaller, and the plus lenses have the opposite effect, they make the object appear bigger. In the unaccomodated eye of the subject the object and the writing appears to be “normal” to them. The issue may occur when a subject never worn correction glasses before and they will have the difficulty on adjusting with the lenses, hence they will seem the objects are too big or too small and the writing is too sharp for their eyes.
An essential role at the collection is to reassure the subject and give the right advice regarding the lenses they need to wear. Therefore, it may be mentioned to the subject that it needs a period of time for the eye to accommodate to the new prescription. Also, the adjustment of the frames is essential in making sure that the subject’s eye will see thru the optical centre of the lens, which gives the best focus on the lenses.
As mentioned above, plus lenses make objects look bigger. This leads to some phenomena as the brain s trying to compensate. As an example, by looking down, the objects will seem bigger (the feet will appear bigger) and the brain wrongly decides that they must be smaller or closer. This automatic adjustment gives the effect of the patient feeling shorter.
Additionally, if one’s hands look bigger, so arms feel shorter. In conclusion, if the brain sees that everything looks bigger, it adjusts and interprets this to give the sensation that they shrunk.
The exact opposite is true for the myopic patient, but because in general as myopia requires correction at a younger age, it is less likely to cause problems in general as younger patients adapt easier to changes than older patients.
A difficulty in adjusting with the glasses can be found in older patients that need to wear correction spectacles for the first time. This group of patients usually require a plus prescription. In addition, patients whom worn contact lenses and never worn spectacles may suffer the same symptoms if they switch to spectacles. These effects will be reduced in time, but nevertheless can be initially disturbing.
One of the complaints that a patient may have is that the magnifying spectacles can make the patient feel sick or giddy. The cause of that is in the brain trying to adapt to and compensate the magnified effect of the lenses.
Various brain reflexes control the eyes and some of the most robust are the Vestibular, which are controlled primarily by the ears. Regardless whether the eyes are open or closed, it is possible to keep the eyes fixed on the same object even if the head is turned or tilted. If this reflex tells the brain that the head is tilted 20 degrees forwards, the eyes will be tilted 20 degrees upwards to compensate, same thing for the left and right movements. However, if the image is enlarged by a plus lens, a 20 degrees upward movement would be enough - if the object looks bigger the eyes would need to make a larger angular deviation to remain “on track”, for about 22 degrees. This can cause the same effect as motion / seasickness, which is caused by a disagreement between what the eyes and the ears tell the brain.
These and other strange perceptions experienced by patients should lessen in time. But they can be disturbing at the beginning. Rarely is necessary to adjust the strength of the spectacle lenses to minimise these unpleasant effects. With a modified correction, the patient may feel more comfortable, although they do not see quite as clearly as with the initial lenses.
As a dispensing optician I have the pleasure of working and dispensing both plus and minus powers and the initial problem that I was faced to was the reassurance that I had to give to the patients when speaking to them about their prescription. On collection, I had encountered patients that were very concerned about that effect on the glasses on the long time wearer. The advice that I gave to them was that he glasses are going to improve their vision and they will help the eyes not to deteriorate. I explained that their vision is improving by wearing glasses and even if they will feel in the first two or three days that their glasses are making them feel a bit strange (e.g. dizzy feeling), they should continue wearing the glasses for at least one week until their eyes adjust to the correct prescription. Also I believe it is important for a dispensing optician to be able to advise the customer by giving them written instruction in how to take care of the lenses and the frames
As a glasses wearer myself, even though I need to wear glasses for concentration only I felt reassured myself when a professional spoke to me in a manner that I understood the importance of wearing glasses and also I understood my prescription.
References:
Dart D. A., Besharse J. & Reza D. (2010) Encyclopaedia of the eye, Available at:
(accessed on 07/12/2010).
Hopkins J.(1996). Epidemiology Reviews, University of Hygiene and public Health, vol. 8, No. 2, Available at: , ( accessed on
07/12/2010)
Jalie M. and Wray L. (2002) Practical Ophthalmic Lenses.3rd edn. Guilford, Surrey: Butterworth &Co Ltd, 1983.
Pears, R.& Shields G.( 2007) Cite them right: a guide to referencing and plagiarism. Newcastle upon Tyne: Pear Tree Books, 2005.
Tunnacliffe, A. H. (2007), Essentials of dispensing. 3rd edn., Godmersham : ABDO College.
