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Progressive Addition Lenses 141

3. Longer and wider progressive corridors
4. More slowly increasing levels of astigmatic error.
In general, harder progressive addition lens designs will provide wider
fields of view, and will require less head and eye movement, at the expense
of more swim and blur. Softer progressive addition lens designs provide
reduced levels of astigmatism and swim while limiting the size of the zones
of clear vision and requiring more head and eye movement.
However, modern progressive addition lenses are seldom absolutely
“hard” or absolutely “soft”. Unfortunately such terms do not accurately
describe modern lenses. Many of the recent progressive addition lens design
incorporates the best balance of these two design philosophies to show
following characteristics:
1. Larger effective distance and near zones
2. Peripheral aberrations are well controlled to enable the wearer to adapt
easily.
3. Combination of hard and soft design.
Recently, SOLA optical introduced another concept i.e. DESIGN BY
PRESCRIPTION the use of different distance designs for different distance
refractive errors. For example, the design employed for the 7.25 base curves
varies slightly from the design employed for the 5.25 base.

OPTICAL DESCRIPTION OF PROGRESSIVE ADDITION LENS
Manufacturers have made various attempts to represent the size and
location of the optical zones and peripheral aberrations of the progressive
lens. Graphically, there are four common ways of progressive addition lens
design representation:

Power Progressive Profile
The primary function of a progressive addition lens is to restore near and
intermediate vision while maintaining clear distance vision. While distance
power and the near vision power in any given lens are fixed parameters,
the lens design must define the progression of power change from one to
the other. Physiological considerations favor a high location of the near
vision zone; however, a short abrupt power progression from distance zone
to near zone will usually create the rapidly varying aberrations in the lens
periphery that cause discomfort. The great challenge for the designer is to
manage both the length of the power progression and the rate of power
change in order to create comfortable exploration of distance, intermediate
and near visual fields without excessive, and tiring vertical head movements
and ocular effort. The rate at which the power increases over the progressive
zone is governed by the power law for the design. The power law may be