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134 Ophthalmic Lenses

Fig. 11.9: No line progressive lens

Fig. 11.10: Progressive lens provides clear vision from far to near

Comfortable Intermediate Vision
The progressive zone of the progressive addition lens gives rise to an area,
which provides the clear vision for the intermediate correction. Only in
the early stage of presbyopia, can single vision and bifocal wearer enjoy
clear intermediate vision, as they can still accommodate and adjust their
head position. But for higher additions, progressive addition lens continues
to offer clear vision at intermediate distance also. Trifocal lens, despite their
clear intermediate field of vision, is not ideal, as the wearer must cope with
the image jump at the two segment lenses (Fig. 11.10).

Continuous Support to the Eyes Accommodation
In a single vision-reading lens, the eyes accommodation is supported for
near vision only. In a bifocal lens, the eyes accommodation experiences
abrupt changes when the gaze shifts from distance to near vision across
the segment lines, because the wearer must constantly choose between
distance and near vision power and switch from maximum to minimum
amplitude of accommodation. For example, consider an eye focusing