Canon celebrates 50th anniversary of lens employing synthetic fluorite

United Kingdom, Republic of Ireland, 07 November 2019 – Canon Inc. and Canon Optron, Inc. today announce 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.

A synthetic fluorite crystal

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

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 2006, 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: Characteristics of fluorite

When light meets water or other transparent matter, it is refracted. Lenses are used to take advantage of this property and focus light that passes through them. However, the degree to which light is refracted depends on the color. For example, blue light, which has a short wavelength will be refracted at a steeper angle than red light, which has a long wavelength. For that reason, even light from the same source will be split by color when it passes through a lens and focused on to various different points, leading to the occurrence of color blurring known as “chromatic aberration.”

An image captured with a telephoto lens where chromatic aberration
can be seen around the edge of the branch in the red square

Chromatic aberration occurs when convex and concave lenses are inversely oriented. It is possible to offset the aberration by combining a small-dispersion convex lens with a high-dispersion concave lens to align the direction of light rays with different wavelengths (such as red and blue) together at the same focal point. However, even with lenses that correct chromatic aberration, if you look closely at the focal point, the intermediate green light between the red and blue wavelengths will still converge at a different point. This chromatic aberration, which remains even after chromatic aberration correction design measures are carried out, is called secondary chromatic aberration, or secondary spectrum. The suppression of this secondary spectrum is where fluorite comes into play.

Compared with optical glass, fluorite lenses have a considerably lower refraction index, low dispersion and extraordinary partial dispersion, and high transmission of infrared and ultraviolet light. Achromatic performance can be achieved by making a convex lens with fluorite, rather than optical glass, to decisively reduce secondary spectrum, causing red, blue and green light rays to almost completely converge on the same focal point to significantly reduce the occurrence of chromatic aberration.

Optical characteristics of optical glass and fluorite

The effects of secondary spectrum are more pronounced for telephoto lenses due to their long focal lengths. To that end, Canon employs fluorite in such contemporary lenses as the EF400mm f/2.8L IS III USM and EF600mm f/4L IS III USM (both released in December 2018). This series of telephoto lenses employing fluorite has earned strong support from photographers worldwide for their subtle rendering and high contrast.