Fluorite
Fluorite is an amazing mineral that emits and disperses light when heated to high temperatures. The stone was named “fluorite” for its beauty and fluorescent properties. Calcium fluoride (CaF2) is a naturally occurring crystalline mineral. When made synthetically, calcium fluoride crystals have outstanding optical characteristics including low dispersion, an extremely low refractive index, and excellent transmittance of infrared and ultraviolet light. Most importantly, fluorite can produce clear, detailed image delineation that conventional optical glass simply cannot match. Canon set out to leverage these properties when it inaugurated the “F” Program, to develop high-performance camera lenses incorporating fluorite.
Fluorite had been a focus of attention for centuries. In the 1800s, natural calcium fluoride crystals were already being used as objective lenses in microscopes. Later, attempts were made to produce synthetic fluorite crystals for use as lenses in larger instruments such as telescopes. However, technical hurdles were high and many believed it would be impossible to use fluorite in standard lenses. This challenge failed to quell the enthusiasm of Canon researchers, as they took up the task of developing fluorite into a viable optical material for use in high-performance lenses.
Differences in the convergence point of wavelengths of light affect the sharpness of an image transmitted through a lens, and appear in photographs as colour bleed. This is technically known as “chromatic aberration.” The key to designing a high-performance lens is to find configurations that correct chromatic aberration. Typically, a low-dispersion convex lens and a high-dispersion concave lens are used in combination to standardise the direction of light waves and align them all at a single convergence point.
However, careful examination of the area surrounding the convergence point of such a lens will reveal a residual aberration in the focal point for green wavelengths, which disperse between the red and blue. This slight residual chromatic aberration is known as secondary chromatic aberration or secondary spectrum aberration. Fluorite is characterised by extremely low dispersion, and unlike optical glass, its dispersion is exceptional. Therefore, it can play a significant role in eliminating this persistent secondary spectrum aberration. When a convex fluorite lens is used, to reduce chromatic aberration in the secondary spectrum, the red, green and blue focal points converge almost perfectly, at a single point. In 1968, two years after the launch of the F Program, Canon researchers successfully produced a synthetic fluorite crystal.
But numerous hurdles remained before fluorite could be used in a camera lens. Because fluorite cannot be ground in the same manner as optical glass due to its fragility, Canon drew upon its legacy of lens grinding technology to develop a special technique for handling fluorite. This grinding process takes four times longer than standard techniques, and afterward, each lens needed to be washed by hand.
In 1969, Canon finally succeeded in producing a lens from fluorite — the FL-F300mm f/5.6 was the world’s first* camera lens to incorporate a fluorite element. Because longer focal length makes telephoto lenses more susceptible to the effects of secondary spectrum aberration, fluorite made a significant contribution to the performance of this lens. Today, the Canon L-series of fluorite-based super-telephoto lenses, characterised by refined delineation and exceptional high contrast, have earned a loyal following among photographers all over the world.
*Refers to lenses for general consumer-use, interchangeable-lens cameras.