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148 Ophthalmic Lenses

Fig. 11.25: Binocular vision with progressive lenses
In designing the following zones of the progressive addition lens, the
lens designer works towards respecting these physiological functions:

Vertical Location of the Near Vision Area
Higher position of near vision area relieves strain to extra ocular muscles
and eases binocular function with downward gaze. But a shorter
progression usually results in rapidly varying peripheral aberrations. A
good compromise consists in locating the usable near vision at a downward
gaze position of about 25°.

Fig. 11.26: Progression of power in relation to head posture
and downward eye movement

Power Progression Profile
A suitable power progression along the meridional line enables the wearer
to explore the object field without tiresome vertical head movements. This
is achieved by associating the shape of the power progression to the