Using focal length of 100mm f/2 as example Double gauss: Double gauss performance: Sonnar performance: Datasheet of sonnar f/4 13.5 cm Zeissikonveb : If you look at the data sheet above, you will be amazed: no heavy or even very heavy crown is needed, no barite flint or similar "exotic" glasses. The two front convex lenses are made of simple boron crown BK7 - one of the cheapest crown glasses that can be produced in large quantities. However, it has decisive advantages if you do not need particularly high refractive indices: it only splits the light into its primary colors to a small extent, which makes chromatic correction easier. Since the 1890s, it had become the standard material for prisms due to its high transparency and, due to its low thermal expansion, it was also often used as a base for mirrors. Only the subsequent flint glass F9 was relatively new in the glass catalog. The result is barely perceptible spherical zones and a practically completely flat image field