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Progressive Addition Lenses 135
Fig. 11.11: Extreme right picture shows clear intermediate vision through
progressive lens
through a bifocal lens at an object moving towards it from a reasonable
distance. The eye first uses the distance power, accommodating to its
maximum amplitude to focus as the object is drawn within arm’s length.
The eye then switches to the near segment where it must totally relax its
accommodation for intermediate viewing, before again increasing to its
maximum amplitude of accommodation as the object is drawn closer. Thus,
the eye varies its amplitude of accommodation twice from minimum to
maximum. This adjustment in accommodation would occur only once with
a progressive lens, just as it would with natural, non-presbyopic
accommodation.
Continuous Perception of Space
Progressive lens also offers continuous perception of space. When an eye
looks through a progressive lens, its perception of spatial relationship is
uninterrupted. Changes of power are gradual and continuous in all
direction with minimum distortion. Single vision reading lenses do not
allow real spatial perception. The two portions of bifocal lenses split and
alter spatial relationship. Vertical and horizontal lines appear broken and
image jump hampers wearer’s vision (Fig. 11.11).
Fig. 11.12: Perception of form and movement through a progressive lens