|Symmetrically designed progressives were the forerunners of today’s progressive lenses. The progressive surface is structured symmetrically, i.e. the major reference points for distance and near vision lie vertically on a line one below the other on the mean perpendicular of the lens.|
To obtain the decentration of the near zone required for convergence, symmetrical progressives have to be rotated by 8° to 10° before being inserted in the frame. This is the only way of ensuring that the eyes can fully utilize the progressive and near zones when they converge.
A major drawback of rotated progressive lenses is the different visual clarity experienced by the right and left eyes when they change their line of vision. In peripheral vision it may happen that the two eyes look through areas of the lens with different image quality, hence severely limiting the field of view usable for binocular vision. This can be particularly noticeable when driving.
PR, PL: visual points in peripheral vision through spectacles containing symmetric progressives
Visual impression obtained in peripheral vision through spectacles containing symmetric progressives
Thickness reduction prism
Engravings and stamp
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