Originally Posted by
Darryl Meister
Some ODs over-plus or "bump" the add power with progressive lenses for one of two reasons:
1. Years ago, the old Varilux II lenses were introduced, which produced excess plus power because of the asphericity in the periphery of the lens. Consequently, some advocated increasing the add power in conjunction with extra minus in the distance power to compensate for this effect. Many "old-timers" might still take this approach. However, lenses are no longer designed like this, so it is not necessary today.
2. Many eyecare professionals also increased the add power with progressive lenses in an attempt to allow the patient to realize his/her full add power at a shorter distance from the fitting cross, particularly in small frames. In a situation where the minimum fitting height cannot be achieved, the patient may be better served by having the frame lowered slightly, selecting a slightly larger frame style, or of course by using a progressive lens style designed for smaller frames -- like AO Compact.
There are several good reasons not to bump the add power with progressive lenses:
1. At least one study has shown that more patients prefer the prescribed add power for progressive addition lenses to the over-plussed add power.
2. Increasing the add power effectively shifts the patient’s area of clear vision up from the near zone and into the progressive corridor, which is often the narrowest region of the lens.
3. Increasing the add power increases the level of unwanted blur in the periphery of the lenses and narrows the areas of clear vision through the lens.
4. Increasing the add power beyond what the patient requires needlessly restricts the patient’s depth of field -- the range of viewing distances through which the patient can clearly see his/her reading materials at near.
Best regards,
Darryl
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