I'm pretty sure they didn't have lenses. Lenses need optically clear glass. Clear glass came along around 100AD, but it took another 1000 years before it was sufficiently clear that you could make good lenses with it.

Glassmaking would be a good one because the chemistry is based on available materials and with sufficient knowledge you would walk them through the history of glassmaking up to a simple lens and make eyeglasses.