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140 Ophthalmic Lenses

In case of symmetrical progressive addition lens design, the right and
the left lenses are identical. To achieve the desired inset for the near zone,
they are simply rotated by an equal and opposite amount in the two lenses,
i.e., 10° anti-clockwise in the right lens and clockwise in the left lens. The
principal drawback of this design is the disruption of binocular vision as
the wearer gazes laterally across the lens, since the astigmatism differed
between the nasal and temporal sides of the distance zone.
Asymmetric progressive addition lens design incorporates a nasal offset
of the near zone and has separate design for right and left lens. So, there is
no need of lens rotation in this case. This results in same peripheral optical
characteristics and better adaptation, improved binocular vision, more
visual comfort and better convergence.

Hard and Soft Design

Fig. 11.17
A harder progressive addition lens design concentrates the astigmatic
error into smaller areas of the lens surface, thereby expanding the areas of
perfectly clear vision at the expense of higher levels of blur and distortion.
Consequently, harder progressive addition lens generally exhibits four
characteristics when compared to softer designs:
1. Wider distance zones
2. Wider near zones
3. More narrow and shorter progressive corridors
4. More rapidly increasing levels of astigmatic error.
A softer progressive addition lens design spreads the astigmatic error across
larger areas of the surface, thereby reducing the overall magnitude of blur
at the expense of narrowing the zones of perfectly clear vision. The
astigmatic error may even encroach well into the distance zone.
Consequently softer progressive addition lens generally exhibits four
characteristics when compared to harder designs:
1. Narrower distance zones
2. Narrower near zones