Semi-finished glass blanks are available with a plano base curve, plastics as
flat as +.50. The glass version would be appropriate for high minus and bonding
to a diving mask. But a minus two on a plano or +.50 BC will have about .50 D of
power error and marginal astigmatism about 15mm off axis- right in the middle of
the reading zone. Use the "best form" six base unless you're bonding to a mask.

Robert Martellaro
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Optician/Owner
Roberts Optical
Wauwatosa Wi.
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"Science is a way of trying not to fool yourself."
- Richard Feynman