Canon's 1967 Fluorite Crystals Fix Telephoto Color Errors
Canon has grown synthetic fluorite crystals since 1967 to correct a specific color fringing in long lenses that ordinary glass cannot fix.
Canon researchers pulled the company's first synthetic fluorite crystals from an electric furnace in March 1967. Two years later, those crystals shipped inside the FL-F300mm f/5.6 lens, which went on sale in May 1969 with two elements of grown calcium fluoride.
Long telephoto lenses carry a specific color error that ordinary optical glass cannot remove. Canon has never stopped growing the material to address this problem, known as secondary chromatic aberration.
What Glass Leaves Behind
A standard achromatic lens doublet corrects for two wavelengths of light, typically red and blue. Everything between and outside those wavelengths still misses the focus point. Canon's own materials call this leftover the secondary spectrum.
On the camera sensor, it appears as a soft violet or green halo around bright edges. It can also manifest as a mild veiling haze that reduces contrast, or as a purple ring around stars in night sky photography.
The size of this residual error is directly tied to focal length. For a given pair of glasses, doubling the focal length doubles the leftover defocus. What lands on the sensor scales with the diameter of the lens's entrance pupil. A 600mm f/4 lens, with its 150mm pupil, spreads secondary spectrum over a blur roughly four times wider than a 50mm f/1.4 lens does.
Nikon's materials-technology pages make the same point from the design side, stating that the longer the focal length, the more difficult chromatic aberration is to correct. This is why the problem belongs to telephotos. At 35mm, secondary spectrum is not a major concern. At 400mm and 600mm, it is the aberration that determines whether a lens looks sharp.
Why Fluorite Works
Plotting the optical properties of standard glasses shows they fall on a nearly straight line. Two glasses from this line can be combined to null two wavelengths, but a third wavelength lands off-target. Fluorite sits well off this line.
Optical-grade calcium fluoride has a low refractive index and weak dispersion. The key property is what Canon calls extraordinary partial dispersion. From red through green, fluorite disperses light similarly to glass. From green through blue, it behaves differently.
This deviation is the free parameter a lens designer needs. Pairing a positive fluorite element with a high-dispersion glass negative allows a third wavelength to be brought onto the same focus as the first two. A lens corrected for three wavelengths is an apochromat.
Fluorite entered optics long before Canon. Zeiss shipped the first apochromatic microscope objective in 1886, calculated by Ernst Abbe, and natural fluorite was part of its construction. Canon's history notes that natural fluorite occurs only in sizes suitable for small equipment like microscope objectives.
The company describes its FL-F300mm f/5.6 as the world's first lens for interchangeable-lens consumer cameras to employ synthetic fluorite. The claim covers a manufactured crystal reaching a consumer camera lens.
Growing Crystals at 1,400 Degrees
Natural fluorite crystals are small and contain color-causing impurities that ruin them optically. They were useless for an element large enough for a 300mm lens. Canon's solution was to grow its own.
The program, called the F Plan, started in August 1966. The first crystals came from an electric furnace in March 1967. Production technology was established by February 1968.
The process involves crushing and refining natural fluorite ore to remove impurities. The material is poured into a graphite crucible, placed in a crystal-growing apparatus with a heater above it, and taken to 1,400 degrees Celsius in a vacuum. The crucible is then lowered slowly so the melt crystallizes from the bottom upward as it leaves the hot zone.
The resulting crystal is annealed over several weeks. It is held at a high temperature just short of melting and then cooled back to room temperature slowly enough to prevent internal strain from cracking it.
| Lens Model | Release Date | Fluorite Elements | List Price (Yen) | Key Specs |
|---|---|---|---|---|
| FL-F300mm f/5.6 | May 1969 | 2 | 100,000 | 7 elements, 6 groups, 3.5m min focus, 58mm filter, 850g |
| FL-F500mm f/5.6 | June 1969 | 1 | 192,000 | Claimed 100 lines/mm resolution at frame edge |
The FL-F300mm f/5.6 featured a green ring on the barrel, chosen to echo the glow that gives fluorite its Japanese name, hotaru ishi, or firefly stone. In December 1974, Canon spun the crystal-growing operation out as Optron Inc., now Canon Optron, which still grows the material.
The Material's Drawbacks
Fluorite is soft, scoring a 4 on the Mohs scale, which puts it below apatite and well below quartz. It scratches easily. It is also brittle, cleaving cleanly along specific crystal planes. A knock that might chip a glass element can split a fluorite one.
Temperature presents a sharper problem. Calcium fluoride has a thermal expansion coefficient about 2.7 times greater than common optical glass like N-BK7. A fluorite element changes size more than twice as fast as the glass around it as the lens warms.
Its refractive index also changes with temperature, moving in a negative direction, unlike most optical glasses. The practical consequence is focus shift. A supertelephoto lens that has been sitting in the sun will not focus at the same plane as it did in the shade. A design with large fluorite elements must be engineered to keep that shift within tolerance.
The material also dislikes thermal shock, which is why moving a large white lens from an air-conditioned car into humid summer air is a slow operation. Canon paints its big telephotos white for thermal reasons. A barrel that reflects sunlight holds the optics closer to a stable temperature, which is especially important for lenses with large fluorite elements.
Machining is another challenge. Fluorite cannot be ground and polished using standard glass processes. Canon says it developed a dedicated method that can take up to four times as long as polishing ordinary optical glass. Combined with the crucible growth, multi-week anneal, yield losses, and final inspection, this creates a supply chain that produces elements at a fundamentally different rate and cost than a glass melt does.
