I look for optical clarity. I had a pair if Maui Jims with glass lenses for 7 or 8 years and loved them. They were tough as nails and remained scratch free. I even forgot them on top of my car once and they flew off at speed. Little ding in the frame but lasted for quite a while after that. Only recently did I open my backpack to find one of the lenses cracked. It was a sad day.