The earliest lens glass was taken from the glass used to make windows at that time. Because its shape is somewhat similar to a crown, it is also called crown glass. Due to the backward process, no two formulas of the lens glass are the same. It’s like the same dish, different cooks fry it, the taste is different.
Later, in order to make achromatic lenses (achromatic lenses), people needed a glass with higher dispersion and higher refractive index. So flint glass, which was originally used for tableware, was chosen as the lens. Compared with crown glass, flint glass is doped with lead oxide, which greatly improves the refractive index and dispersion of the glass.
Due to limited conditions, the above crown glass and flint glass were the only lens glass that lens makers could touch at that time. But just by using these two kinds of glass, people can already make a lens that eliminates chromatic aberration!
The texture of the early lens glass was very uneven, and it often contained air bubbles. In 1774, a Swiss named Pierre-louis guinand discovered that by stirring a glass solution, a more uniform glass could be produced. But he chose to keep this secret for many years and did not make a fortune.
The earliest and most successful optical glass company in the world was Schott Glassworks founded by Ernst Abbe and Otto Schott in Jena, Germany, which is today’s SCHOTT AG company. In just six years, they have developed 44 types of optical glass. The most famous of these is the high refractive index glass doped with barium. People like glass with a high refractive index, because the higher the refractive index, the stronger the ability to condense light. This kind of glass quickly dominated the optical glass market for decades, until the invention of rare earth element glass.
In 1930, the American Kodak Company developed a glass with a higher refractive index than the barium reference glass. This time the glass is doped with rare earth elements lanthanum and thorium. Generally high refractive index glass, its dispersion is also very high. The optical glass containing rare earth elements not only has a high refractive index but also maintains a low dispersion. An optical glass containing lanthanum is used in almost all camera lenses today.
It is worth mentioning that the early rare-earth glass type was radioactive. From the 1940s to the 1970s, thorium-containing glass was widely used in lens manufacturing. If you have an old Kodak lens, it is probably radioactive. But you don’t have to worry too much, as long as you are one meter away from it, its radiation dose is almost negligible. Another disadvantage of this kind of glass is that it turns yellow over time. This is caused by the decay of radioactive elements inside. The solution to this problem is also very simple, just take a shot in the sun.