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Progressive Addition Lenses 151

The amount of prism thinning used is roughly equal to the 2/3rd of the
addition power. This is often recommended when the power through the
vertical meridian of the lens exceeds + 1.50 D or so. This formula does not
consider factors like the fitting height and the distance power, but still
produces satisfactory results in most cases. It is possible to prism thin the
minus powered lenses as well. Depending upon the fitting height either
base down or base up prism may be required to balance the thickness
difference.
We can verify this prism onto the progressive addition lenses by placing
the prism reference point of the lens in front of the centre of the lens of the
focimeter. This is very important especially where only one lens is being
replaced. If the previous lens had not been prism thinned and the new one
was or vice-versa – an unwanted vertical prism imbalance will be induced.
In summary prism thinning is a useful tool that improves both the finished
cosmetics of many progressive addition lenses with little visual impact to
the wearer.

LIMITATION OF CONVENTIONAL PROGRESSIVE ADDITION LENSES
With all the advancements from hard to soft design and from a required
minimum fitting height of 26 mm to 14 mm, many eye care professionals
assumed that progressive addition lenses had come relatively close to
perfection. However, this is not true. There are still four basic limitations
in conventional progressive addition lenses created by their inherent
designing:

Fig. 11.29: Progressive addition lens magnifies the image in its various section

1. Differing Magnification throughout the Lens
The changing curves on the front lens surface and the change in power
through out the channel and reading portion of the lens create varying
magnification through out the lens. The magnification increases throughout
the progressive zone. The result is that the vertical lines viewed through
the progression zone exhibit skew distortion.