Page 8 - Cahier-Progressive Lenses-GB.indd

Concept B Advantages of Progressive power lenses Compared with single vision and bifocal lenses, progressive 1 lenses offer the presbyope the following advantages : Continuous vision from distance to near : single vision len- ses offer a range of clear vision in the near area only whe- reas bifocal designs, with their abrupt change in power, create two ranges, one for distance and one for near vision. Comfortable vision for all intermediate distances (50 cm to a) the distance zone 1.5 m) since the progressive lens is the only design which Progressive lenses clear vision at these distances. A low reading addition, has zones specifically designed for these distances. In the of a bifocal lens. early stages of presbyopia (additions less than 1.50 D), the wearers of single vision and bifocal lenses may still obtain together with their remaining amplitude of accommoda- tion, enables them to see clearly over this range. On the other hand, in later stages of presbyopia (additions over 2.00 D), clear intermediate vision is no longer possible since the remaining amplitude of accommodation is too little, and the reading addition too great, to provide clear vision at these distances. Only progressive lenses allow comfortable vision at all intermediate distances. Continuous support for accommodation adapted for all distances : with a progressive lens the eye can find some region of the progression zone where the power is correct for the viewing distance. In the case of a single vision rea- ding lens, vision is possible only for near vision. With a bifo- b) the near zone of a cal lens the eye must increase and then relax its accom- bifocal lens. modative effort as it passes through the intermediate range. Continuous space perception is obtained from the gradual change in power across the lens. Single vision lenses do not provide real space perception since they limit vision only to the near range. Bifocal lenses divide the range into two whilst altering perception in that horizontal, and vertical lines appear broken and produce image jump at the divi- ding line between distance and near. c) the intermediate zone of a progres- sive lens. Limitations of progressive lenses : Although progressive lenses possess many advantages they also have some limitations. In effect, due to the laws of physics, optical aberrations arise as a result of the necessary power variation across the surface. For this reason all progressive lenses have undesirable Fig. 4 : Vision at intermediate distances changes in spherical and cylindrical power in their lateral zones. The art of the designer is to manage and control these aberrations in every possible way taking into account the physiology of vision and using all available means for lens design and calculation. 8 Copyright © 2006 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.