A couple of threads over the years have been devoted to inexpensive backup glasses; recently I purchased a pair from Zenni and have been very pleased with them. Well, from now til Sunday (Sept 23rd) they have a deal going on where you get a second pair free with the purchase of a pair of glasses. Again, the standard disclaimers- I'm not affiliated with them in any way beyond being a satisfied customer.

I think I'm gonna pick up a pair of Ti framed glasses!
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman