The advantages and disadvantages of bifocal lenses
The bifocal glasses present three main disadvantages: the jump of the image when the visual axis passes from the far vision glass to the reading segment, the prismatic effect on the near vision point that entails an apparent displacement of the fixed object as well as a degradation of the quality of its image, and the absence of intermediate vision in total presbyopia. Bifocals however keep their indications, even after the availability of the progressive glasses, by people accustomed with this kind of correction, and by others who are not supporting deformations in the lateral vision of the progressive glasses. Suggestions are made for choosing the model of bifocal which is the most appropriate in function of the different situations.
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- "It would be similarly unethical to compare bifocal and varifocal lenses as generic products against single vision lenses for most wearers. However, despite the clear commercial success of the products, even the peer reviewed evidence for preference of varifocals over bifocals (Borish and Hitzeman, 1983; Thorn and Baker, 1984; Boroyan et al., 1995; Markovits et al., 1995; Krause, 1996) is far from conclusive (Hitzeman and Myers, 1985; Krause, 1996; Zanen, 1997). There is only anecdote to show that bifocal use has not declined in the UK solely as a result of the death of existing wearers, the retirement of doubting opticians, and the pro-active and more profitable selling of varifocals by practitioners made more confident by success itself. "
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ABSTRACT: The evidence for the basic optometric function of optical dispensing is discussed in terms of the basic principles of scientific debate. The degree of rigour applied to product testing and evaluation of most other medically related areas is lacking in the dispensing of spectacles. There has been much debate on the place of religious faith in our society. Religious belief masquerading as science has come in for particular scrutiny. However, little attention has been paid to the equally contentious issue of non-religious belief, within scientific disciplines, masquerading as science. It could be argued that optical dispensing is, to a significant degree, such a case.
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ABSTRACT: Good lighting and the correct use of spectacles generally improve vision and presumably reduce the risk of accidents. However, bifocal and varifocal spectacles can increase the risk of misjudging distances when negotiating underfoot hazards. In some circumstances, the portion of the lens used for close work may inadvertently be used where accurate judgement of distance is necessary. This paper reports results from two studies of patient interviews using the Merseyside Accident Information Model; the first a study of accidents which occurred during paid employment (1504 cases) and the second of 1326 accidents mainly in domestic and leisure activities. Underfoot accidents were identified by the first unforeseen event perceived by the patient. The relationship between underfoot events and the type of spectacles worn by the patient at the time of accident was analysed. In both studies there was a significant association between accidents where the first event was ‘missed edge of’ (step) and wearing bifocal/varifocal spectacles. Corporal movements reported in both studies indicated that ‘stepping down’ when wearing bifocal/varifocal spectacles, increased the risk of ‘missed edge’ accidents. In the work environment corporal movements associated with manual handling of loads also increased the risk of ‘missed edge’ accidents when wearing bifocal/varifocal spectacles.
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