I purchased these glasses after losing a prescription version of them, so this is my second pair, which should tell you that I really love these glasses.  I originally bought them at the recommendation of my Optometrist since I am involved in boating, sailing, fishing, and snow skiing, all which reflect a great amount of sunlight.  These glasses are designed to block out sunlight not only through the lenses, but the light that typically enters around the frames, so when you are wearing them, you get superior protection from the sun.  My first pair had plastic 400-series lenses, while my second pair has the glass 580-series lenses.  The plastic lenses are needed if you are getting a &#34;progressive&#34; prescription, which is what I had, and the plastics are supposedly lighter.  I really don't notice any difference in weight with the glass lenses (they don't feel heavy), but they are noticeably clearer and supposedly more scratch resistant than the plastic (per the Costa del Mar website).  There is a definite difference in the wavelength of light that gets through the 400 and 580 series lenses and according to the Costa del Mar website, the 580's block blue and yellow light to cut haze.  To me, the 580 lenses definitely seem clearer than the 400 lenses, but I also went from plastic (400P) to glass (580G) lenses, so it's hard to tell if it was the change from 400 to 580 or from plastic to glass.  In any case, the Tuna Alley glasses with the 580G lenses are awesome.  They are comfortable (I have forgotten that I am wearing them and started to look for them!) and provide incredible sun protection.  I personally think that the crystal frames with the blue finished lenses are super cool looking and I get lots of comments on them, but they come un a number of colors.  While they don't really need a retainer to hold them in place, I would recommend one if you are fishing or boating.  I lost a pair by putting them up on my baseball cap after sundown while cleaning the deck- not a good idea.  I bought the Chums retainer with the large ends which work perfectly.