was looking up what the K9 optical lens is and found some info - I hope the Z1 has this glass:
What is K9 Crystal Glass?
K9 is a Chinese Optical Borosilicate Crown Glass. It has very low inclusions and bubbles. It is used for lenses and prisms for lasers and optics. It has excellent physical properties and can be polished like leaded glass. It also weighs approximately 15% less than full (24%) leaded glass (crystal). The US equivalent is BK7.
Crystal glass: Manmade crystal glass always comtains high-percentages lead .Since the hardness and refraction is dictated by the PbO percentage, certain amounts of PbO are often added into general glass, and when above 30%0, the refraction will be higher than glass and crude crystal. In the domestic (Chinese) market, crystal glass means K9 optical glass. In 2000, the first China Crystal glass exhibition commission searched and registered this technical expression at the State Bureau of Light Industry, which exactly described the crystal material in the domestic market.
Products made of K9 are extremely popular because of the high quality, durability and reasonable price.
