Buying new spectacles probably isn't high up on your Běijīng bucket list, but when you can get your eyes tested and lenses cut inside about forty minutes (for around ¥200), not to do so would seem a little short-sighted, surely? Five sprawling floors of glasses sellers (contact lenses are up top) equals a vast choice of frames and styles, including prescription sunglasses.
If you don't know your prescription, they can obtain it by scanning your existing glasses, or you can take an eye test. Note that in lieu of an alphabet, Chinese eye tests use what looks like versions of the letter 'M' flipped so that its legs face in any of the four cardinal directions. You simply have to say up (shàng), down (xià), left (zuǒ) or right (yòu). Failing that, just point. It's not a tourist trap like the Silk Market, though bargaining might be necessary for foreign faces.