This slight residual chromatic aberration is known as secondary chromatic aberration or secondary spectrum. Fluorite can play a significant role in eliminating this persistent secondary spectrum. Fluorite is characterized by extremely low dispersion, and unlike optical glass, its dispersion is anomalous. By creating a convex fluorite lens to eliminate chromatic aberrations, secondary spectrum becomes extremely small, and red, green, and blue focal points converge almost perfectly. The focal points converge at a single point, creating the image sharpness that puts Canon L-series lenses in a class of their own. In 1968, two years after the initiation of the F Program, Canon researchers grew a synthetic fluorite crystal. But numerous hurdles remained before fluorite could be incorporated into a camera lens. Because fluorite cannot be ground in the same manner as optical glass, Canon relied on its legacy of lens grinding technology to develop a specialized technique for working with fluorite.
This grinding process took four times longer than standard techniques, and afterward, each lens needed to be washed by hand. In 1969, we finally succeed in producing a lens using fluorite – the FL-F300mm f/5.6, Canon’s first camera lens to incorporate a fluorite element. Because longer focal length makes telephoto lenses more susceptible to the effects of secondary spectrum, fluorite made a significant contribution to the performance of this lens. Today, the Canon L-series of fluorite-based super telephoto lenses, characterized by refined delineation and exceptional high contrast, have earned a loyal following among photographers all over the world.