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138 Ophthalmic Lenses

To understand how the length of the progressive corridor and the add
power can affect the rate of change and magnitude of the astigmatic error,
consider a progressive lens with a + 2.50 D addition, which has a 17 mm
progressive corridor. This lens will have to change by 2.50 over a distance
of 17 mm. This implies plus power represents an average change of roughly
2.50/17 = 0.15 D per millimeter. Therefore, the power of a progressive lens
changes more rapidly down the progressive corridor as the addition power
increases or the length of the corridor decreases.
A well designed progressive lens will reduce the amount of astigmatic
error to its mathematical limits for a given design. During the design and
optimization process, various parameters are adjusted to control and
manipulate the distribution and magnitude of this astigmatic error across
the progressive lens surface. The width of the near and distance zones, and
the length of the progressive corridor are the chief parameters that are
altered.
The magnitude, distribution and the rate of change (or gradient) of this
astigmatism error are all performance factors that can affect the wearers
acceptance of the lens. The amount and gradient of peripheral lens
aberration determines the field of view, while the gradient and type of
aberration primarily influence adaptation time. A steeper gradient
concentrates the change in aberrations over a smaller distance and provides
wider distance and near zones free of aberration. Astigmatic aberration is
generally perceived as distortion or blur while prismatic aberration is
perceived as “swim” or “waviness” with head movement.

Fig. 11.15: Power profile for a progressive lens

PROGRESSIVE ADDITION LENS DESIGNS
Basically we can categorize the progressive addition lens design into three
groups:
• Mono design and multidesign
• Asymmetry and symmetry design
• Hard and soft design.