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Ophthalmic Lens Material and Design 25

higher powers in plus to steeper front curves and while moving greater
minus powers to flatter front curves.
The rule of thumb for selecting an appropriate base curve is take the
spherical equivalent of the given prescription, if it is plus – add + 4.00 to
that power to get a good approximate base curve. If it is minus – add + 8.00
to that power to get a good approximate base curve.
Example—Given prescription + 4.00Dsph + 1.00Dcyl × 90
Spherical equivalent + 4.50D
Base curve + 4.50 + (4.00) = + 8.50D
Given prescription – 4.00Dsph – 1.00Dcyl × 90
Spherical equivalent – 4.50D
Base curve (– 4.50D) + 8.00 = + 3.50D

However, while selecting a proper base curve, it is necessary for the
dispenser to utilize knowledge of theory, practical availability and common
sense. To simplify the concept further we can say that as minus power
increases the selection of base curve flattens and as plus power increases
selection of base curve steepens. Care has to be taken when changing the
lens material from lower index to higher index material. In order to do a
professional job, the dispenser must monitor all the options and then make
the decision:
1. Determine the old prescription.
2. Determine the old curve what the wearer is used to.
3. Evaluate the new prescription in accordance with theoretically ideal
curve.
4. Determine the best lens design and availability.
5. Check that the design could be fabricated by the laboratory or not.
Basically the gist is “just pay attention”.

SUMMARY
All these current base curve theories are based on spherical lenses, and
they do not apply to the thinner and flatter aspheric designs that are
becoming a staple in today’s industry. This enables the lens designer to use
the combination of bending and asphericity to be employed to improve
the off-axis performance. As a result, the designer can choose any form for
a given power and then determine the asphericity required for that
particular choice of bending to eliminate oblique astigmatism.