Canon celebrates 50th anniversary of lens employing synthetic fluorite

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The FL-F300mm f/5.6 (released in May 1969), the first Canon ILC lens to employ synthetic fluorite

Canon Inc. and Canon Optron, Inc. announced the 50th anniversary of the release of the FL-F300mm f/5.6 (released in May 1969), the world’s first lens for consumer interchangeable-lens cameras which employed synthetic fluorite. Alongside Canon’s interchangeable-lens range, synthetic fluorite is also used in a wide selection of Canon’s optical products, including broadcast lenses and astronomical telescope lenses.

When combined with optical glass, fluorite – the mineral form of calcium fluoride (CaF2) – can correct chromatic aberration to such a degree that it is near non-existent. However, naturally occurring fluorite crystals are too small to be of practical use in photographic lenses. To create the vivid, subtle rendering – not possible with typical optical glass – Canon turned its attention towards effectively harnessing fluorite early on. The Canon F Plan commenced in August 1966 with the aim of developing a high-performance lens that employed fluorite, setting in motion the company’s commitment and development of high-performance lenses.

In 1950, a technique for synthesising fluorite with naturally occurring fluorite crystals was discovered, which paved the way for its use as an optical material. However, a vacuum environment at temperatures above 1000 degrees Celsius was required to cultivate fluoride crystals, which caused many in areas such as installation and manufacturing, to achieve the mass production of large-size high-purity crystals.

Fueled with a desire to develop high -performance lenses, Canon succeeded in the first fluorite crystals synthesised in an electric furnace in March of 1967, and in February of 1968, established synthetic fluorite crystal production technology. At that point in time, fluorite could not be polished like typical optical glass. Canon therefore developed non-conventional machining technology capable of polishing the delicate material, which took four times as a long. In May 1969, Canon’s first interchangeable-lens employing synthetic fluorite, the FL-F300mm f/5.6, was released. Since then, the use of synthetic fluorite has been crucial to Canon’s development of high-performance lenses.

In December 1974, Optron Inc. (now Canon Optron) was established to commercialise the fluorite crystal mass production technology Canon had cultivated. Canon Optron developed a variety of other optical crystal materials while refining its high-temperature vacuum and temperature control technologies for the mass production of synthetic fluorite. In July 2007, Canon Optron provided 12 lenses to the Smithsonian Astrophysical Observatory – including a synthetic fluorite 40cm wide lens, capable of registering signals from over 10 billion light years away.

Canon is committed to improving its imaging technologies, with a focus on optical performance, to ensure its imaging solutions meet the high expectations of its users. Canon will continue to develop and manufacture reliable products, with enhanced optical performance, which play such an integral part in society.

Reference: About fluorite properties

When light hits water or something transparent, it refracts. The lens uses its properties to focus light that is transmitted. However, the degree of refraction depends on the color: for example, blue light with a shorter wavelength refracts at a steeper angle than red light with a longer wavelength. As a result, light emitted from the same light source is separated into different colors in the lens, and their focal positions are different. This causes a color bleed called “chromatic aberration”.

Because chromatic aberration occurs in the opposite direction between a convex lens and a concave lens, correction is made by combining a convex lens with a small dispersion and a concave lens with a large dispersion, aligning the direction of travel of the light rays with one, canceling the aberration, and matching the focal point. However, even with lenses combined to correct chromatic aberration, the green focal point, which is halfway between the red and blue wavelengths, still shifts near the focal point. This slight residual chromatic aberration is called secondary chromatic aberration or secondary spectrum. “fluorite” is effective in eliminating this aberration completely.

Compared to optical glass, fluorite lenses are characterised by “very low refractive index”, “low and anomalous partial dispersion characteristics”, “Excellent permeability to infrared and ultraviolet light”, etc. Making use of this characteristic, which is not found in ordinary optical glass, a fluorite convex lens is made, and the secondary spectrum becomes extremely small by achromatisation, and almost all of the red, green, and blue focal points coincide, and the light is focused at a single point, thus achieving almost complete elimination of chromatic aberration.

In super telephoto lenses, which are greatly affected by the secondary spectrum due to their long focal length, fluorite is highly effective. For this reason, Canon has adopted fluorite for its newest lenses such as “EF400 mm f/2.8 L IS III USM” “EF600 mm f/4L IS III USM” and (Both launched in December 2018). The super telephoto lens series using fluorite is highly popular with photographers all over the world for its delicacy of description and high contrast.