Buying new spectacles probably isn't high up on your Beijing bucket list, but when you can get your eyes tested and lenses cut in about 40 minutes (for around ¥200 to ¥300), not to do so would seem a little short-sighted, surely? Four sprawling floors of glasses sellers (contact lenses are up top) equals a vast choice of frames and styles, including prescription sunglasses.
From the subway exit, turn left and cross the main road via the footbridge. Head for the glass building with a giant pair of glasses stuck to it.
If you don't know your prescription, the stores here 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 Silk Street, though bargaining might be necessary for foreign faces.