Photochromic or transition lenses that darken outdoors are useful if you're sensitive to sunlight. They block 100 per cent of the sun's ultraviolet rays, so you don't need prescription sunglasses.

Step 1: Grab your current glasses, your smartphone and a credit card (or any other plastic card that's the size of a credit card -- it's used to calibrate the app, not for payment). Then, plunk down ...

If you're a glasses wearer, you may have wondered "how often should I get new glasses?" You should never ignore your eye ...