To break it down... C means copper in color, a lens tint that offers increased contrast. 12 is the percent amount of visible light it lets through the lens, perfect for full sun because it blocks a lot of light.So that makes C28 a copper colored lens that allows 28 percent of the visible light through - good for lower light conditions than full sun.I have worn C28 in full sun, as the person above has, and found that although it doesn't block as much light as the C12, it is still extremely comfortable. They are both great lenses that offer great clarity, contrast and eye comfort. Whenever I wear either of them I always think "I wish my eyes always worked this good!"