Try this! Take a low power poly or trivex lens(like a -2.00) and cut it to perfect size in a full rim frame on one side.On the other side do the same except leave it a little larger. Look at the lens through a polarscope,then you will see the excessive stress in the lens .This stress can cause distortions in an opthalmic lens.Since trivex "flexes" more than poly the potential for this stress is greater.This unwanted stress can also cause abnormal low abbe values in a given lens.
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