Progressive Lenses as you know combine correction for both Short Sight and Long Sight. But the lenses do not have a separation between the lenses used. They offer a gradual change between the corrective portions of the lens.
The lower portion of the lens corrects the Long Sight and upper portion corrects the Short Sight similar to the conventional bifocal lenses. The middle portion of the lens helps one to see objects at intermediate or middle distances. The power gradually changes. They are also called “No-Line” Multifocal Lenses, Progressive Addition Lenses (PAL), Varifocal Lenses etc.
The Progressive Lenses are characterized by a gradient of increasing lens power starting from top to bottom. The Carl Zeiss AG and Varilux Lenses were the first in the modern Progressive Lens design.
Advantages of Progressive Lenses
- You need only one pair of eyeglasses to see near and far
- The PAL enables the patient to see clearly at all viewing distances.
- Bifocal Lenses normally have an image jump creating discomfort to the wearer. This is not there with the Progressive Lenses
- Since there is not line separating both lenses, the Progressive Lenses are more attractive and appears to be like Single Vision Lenses
Disadvantages of Progressive Lenses
- The Progressive Lenses are more costlier than the normal Bifocal Lenses
- Precise measurement of the separation point must be taken while ordering a Progressive Lens. Otherwise viewing distant objects will be difficult with the normal position of the head. One will have to tilt the head
- There can be aberrations and/or geometric distortion which leads to poor vision while turning the eyes down and to the sides
- There is a learning curve (till one gets adjusted to the new lens) and some people find it difficult to adjust with Progressive Lenses