OPTICAL CORRECTION

Varifocus

Progressive lenses, also called progressive addition lenses, progressive power lenses, graduated lenses and varifocal lenses, are eyeglass lenses used to correct presbyopia and other disorders of accommodation. A gradient of increasing lens power is added to the correction for the other refraction error, going from a minimum or nothing at the top of the lens to maximum magnification at the bottom of the lens. A wearer can then adjust the lens power required for clear vision at different viewing distances by tilting his or her head to place the line of sight through different parts of the lens.

Progressive addition lenses avoid the discontinuities in the visual field created by bifocal and trifocal lenses. The lenses are also more cosmetically attractive. The lenses suffer the disadvantage of creating regions of distortion and blur away from the optic axis, yielding poor visual resolution. Although manufacturers are constantly striving to minimize these distortions, some wearers cannot tolerate the lenses.

Essilor lay a strong claim to have invented varifocals in 1958 with their first design of Varilux lens. Early varifocals were relatively crude designs but modern sophisticated varifocals have gained much greater patient acceptance and include special designs to cater for many separate types of wearer application, e.g. for use on with computers, offering enlarged near and intermediate areas.

The varifocal lens give a progressive vision from near to far, for the PRESBYOPIC eye.

When the designer reduces the amount of astigmatism which occurs in the lower portion of the lens - in an attempt to speed the subject's adaptation to progressive lens wear - by spreading the astigmatism into the distance portion as indicated in the following figure, this arrangement results in a soft progressive design. There can be no doubt that when the addition is low and. hence, the surface astigmatism is low. The soft progressive design has proved to be the most successful in enabling rapid wearer acceptance of progressive power lenses.

Many manufacturers now produce progressive lens series that are deliberately soft in design for the low-addition lenses in the series, the design tending to become harder as the additions increase. These are known as multi-design series.The following figure illustrates how the power law differs with a multi- design series for the additions. + 1.00. +2.00 and + 3.OOD. It is also seen that the length of the progression zone reduces as the addition increases for these lenses.

These features of modern progressive power lenses, together with the attempts to ensure that prismatic effects are similar at corresponding points on the lens. so-called horizontal symmetry of the design, have ensured that the vast majority of wearers will adapt to progressive lens wear.

Traditional progresive lens

•  Mminimum height 24 mm
•  85% of the addition is obtained after 14mm of progression
• No prismatic compensation
• Large field of view for distant vision
• Narrow progression
• Good field for near vision
• Inexpensive progressive lens
• Indication: For old type of lenses wearer

Modern Design

• Mminimum height 24 mm
• 85% of the addition is obtained after 15mm of progression
• Prismatic compensation
• Maximum of aberrations under the add.
• Medium field of view for distant vision
• Good et large progression zone
• Good near vision: Good Progressive lens for small budjet .

Top modern Design

•  Mminimum height 22 mm
• 85% of the addition is obtained after 14mm of progression

• Prismatic compensation

• Maximum of aberrations under the add.
• Large field of view for distant vision
• Good et large progression zone
• Good near vision: Good Progressive lens for all.

Although it is easy to draw some conclusions about the likely performance of one design over another by inspection of iso-cylinder diagrams it is more difficult to pontificate over acceptance of the design from these diagrams. The last figure indicates quite clearly that the soft design has a narrower intermediate channel and a narrower near portion than the lens that is described as a hard design. The widths of these areas could be measured and expressed, for example, in the same way as we would express the diameter of a bifocal segment. However the significance of this information is not immediately apparent. The author wears several different progressive lens designs.

Each with different characteristics in their intermediate and near portions, and has no strong preference {at least, which is related to the optical performance of the lenses for any one design over another.

Needless to say the advantages of progressive designs over other forms of multifocal correction should be spelt out in simple terms.

• These lenses will enable you to focus at all distances.
  To first-time young presbyopes
  They will be easier to get used to than bifocal lenses.
  In wear the lenses will restore the vision of youth.
  There are no tell-tale dividing lines on the lenses.