Fluorite lenses: Corrective capabilities past the bounds of abnormal optical glass
Fluorite lenses: Corrective capabilities past the bounds of abnormal optical glass
One essential materials that helps the excessive picture high quality attribute of Canon lenses is fluorite, which is a crystallized type of calcium fluoride. It has lengthy been recognized that utilizing a fluorite lens at the side of a
glass lens can scale back chromatic aberration to extraordinarily low ranges. Nevertheless, pure fluorite happens in small sizes which can be appropriate just for use in small optical gear reminiscent of the article lenses of microscopes. Canon, in its pursuit
of progress in imaging capabilities, was eager on using fluorite in its pictures lenses, got down to develop its personal know-how for forming giant, high-purity synthetic fluorite crystals utilizing fluorite ore as a uncooked materials. In Might
1969, the FL-F300mm f/5.6—the world’s first shopper telephoto lens to make use of fluorite lens parts—was launched.
How fluorite lens parts appropriate chromatic aberration
You might need observed the outlines of your topics tinted in a manner that resembles a colour fringe. That is chromatic aberration, and it will possibly additionally take the type of a haziness all through the whole picture. As this prevents the topic
from being rendered precisely, it must be corrected in order that the picture high quality is sharp, clear, and trustworthy to the scene.
The purple fringing on the edges if these branches is a type of chromatic aberration.
Chromatic aberration occurs as a result of when mild passes via a glass floor, the different-colored waves inside it (pink, inexperienced, blue, and so forth.) are refracted at totally different angles because of their totally different lengths, with every
colour converging at a unique focus. Such aberration is normally corrected by utilizing a mixture of concave and convex lenses, which refract the sunshine in reverse instructions to one another.
Chromatic aberration correction utilizing concave and convex glass lenses
Nevertheless, it isn’t attainable to appropriate the chromatic aberration on all wavelengths by utilizing abnormal glass. The chromatic aberration, which happens on sure wavelengths reminiscent of pink, is named residual chromatic
aberration. Because the refractive index of the wavelengths differ relying on the kind of optical glass, there are limitations to how a lot residual chromatic aberration will be mitigated relying on the mixture and properties of the
optical glass.
That is the place fluorite turns out to be useful. As fluorite is a basically totally different materials than typical optical glass, it may be utilized in mixture with glass to appropriate chromatic aberration extra successfully. It’s
notably efficient on telephoto lenses, the place the lengthy focal size exacerbates chromatic aberration.
Chromatic aberration correction utilizing a convex fluorite lens and a concave glass lens
Canon’s fluorite lens parts incorporate pure fluorite as a uncooked materials, endowing the lenses with low-refractive, low-dispersion properties not attainable with glass lenses. Fluorite lenses are additionally distinctive of their
extraordinary partial dispersion tendencies: the pink to inexperienced wavelengths are dispersed with the identical tendencies as glass, however the inexperienced to blue wavelengths are dispersed greater than glass. Utilizing a convex fluorite lens factor alongside a
high-dispersion glass concave lens factor due to this fact eliminates residual chromatic aberration, making attainable a lens that produces clear, sharp, and high-quality photos.
Refraction and dispersion
‘Refraction’ is the phenomenon through which mild modifications course when it passes via the floor of a cloth reminiscent of glass. The diploma of the directional change is named the ‘refractive index’. Because the refractive
index varies relying on the colour constituents (wavelengths) of the sunshine, every colour bends in a unique course. This is named colour dispersion. On optical glass, dispersion happens at a set proportion whatever the
wavelength, whereas on fluorite, dispersion happens at totally different proportions for various wavelengths and is named ‘extraordinary partial dispersion’.
The emergence of fluorite lenses and the way they enhance the picture high quality of telephoto lenses
Fluorite lenses transcend conventional limitations to cut back chromatic aberration to an especially low degree. These lenses have their origins the Canon F Venture, which began in August 1966. Canon’s lens builders strongly
believed that to create a lens that was carried out higher than present lenses, it was first essential to create a brand new materials, and it was this conviction that drive them to determine the know-how for producing synthetic fluorite
crystals to make use of in digital camera lenses.
The problem in producing synthetic fluorite crystals lay within the crystallization. The time period “glass” was initially used to explain the state of a cloth. Being non-crystalline, it consists of atoms fastened in random
positions–simply melting glass permits it to be processed into totally different shapes. Then again, fluorite is a crystalline substance, and its constituent atoms should be organized in a particular configuration for it to crystalize.
To pulverize pure fluorite, purify it, after which restore the precise atomic construction to recrystallize it right into a dimension giant sufficient for software in a digital camera lens is a near-impossible feat. Due to this fact, the builders needed to
guarantee a exactly managed vacuum setting the place the temperature is stored at the least 1,000℃, in order that they designed an equipment to artificially produce giant crystals with excessive purity In 1968, two years after the F venture began, Canon
lastly managed to beat these obstacles to efficiently kind synthetic fluorite crystals giant sufficient for use in digital camera lenses.
The stones on the left are pure fluorite crystals. They’re inexperienced and purple because of impurities throughout the crystals. On the center is a man-made fluorite crystal ingot produced by Canon.
When heated, pure
fluorite glows with a dreamy mild that resembles that of fireflies, which is alleged to be the rationale behind its Japanese title, hotaru ishi, actually “firefly stone”. Identical to how fireflies require clear water to stay, solely
pure fluorite can “shine” as pictures lenses.
One other problem in fluorite lens manufacturing is its sharpening. As fluorite is softer and extra delicate than glass, the identical strategies used for sharpening glass usually are not appropriate. Due to this fact, Canon developed a particular know-how
for sharpening fluorite lenses that requires as much as 4 occasions longer than the time wanted to shine abnormal optical glass. This know-how was efficiently commercialized the next 12 months, in Might 1969.
The primary lens to make use of fluorite lens parts was the FL-F300mm f/5.6 (launched in 1969). The intense inexperienced line, meant to evoke the picture of the glow of fluorite, signifies that the lens contains a fluorite
lens factor.
The FL-F300mm f/5.6, whose profitable commercialization was achieved by transcending many challenges, was extremely acclaimed for is vivid, high-contrast rendering, and have become broadly utilized in such professions as
photojournalism.
Canon subsequently improved on is excessive temperature vacuum, temperature management, and sharpening applied sciences, enabling the usage of fluorite lens parts in lots of extra lenses. Canon stays dedicated to pursuing the very best
picture high quality for its telephoto lenses.
The fluorite lens manufacturing course of
Whereas the grinding and sharpening processes could seem equivalent for all types of optical glass, every stage of the fluorite lens manufacturing course of requires gradual, meticulous consideration to element.
1. Uncooked supplies
The uncooked materials for fluorite lenses is of course occurring fluorite ore.
2. Pulverization and refinement
The uncooked fluorite is pulverized and refined to take away impurities earlier than being poured right into a graphite crucible that doesn’t soften simply.
3. Crystallization
The crucible is positioned in a crystal-growing equipment with a heater on prime and heated to 1,400℃. After the uncooked fluorite has melted, the crucible is regularly lowered, permitting crystallization to happen
ranging from the underside of the crucible.
4. Annealing
The annealing course of removes strains that happen contained in the crystals fashioned. Pressure that result in cracks are eliminated by heating the crystals to a excessive temperature inadequate to soften them, after which slowly
cooling them to room temperature over an extended interval of a number of weeks.
5. Trimming and tough processing
The pointless components of the crystal floor are trimmed off, and the crystal is rough-processed to the required dimension. The inside of the crystal is inspected for anomalies.
6. Grinding
The highest and backside surfaces of the crystal are floor right into a spherical form with a floor that resembles frosted glass.
7. Sprucing
The surfaces of the crystal are polished with a pellet comprised of coagulated polish till they’re semi-transparent and meet the desired dimensions. Lastly, a particular polish is used to take away advantageous scratches.
8. Vapor deposition
Coating materials is heat-evaporated underneath excessive vacuum situations as much as one-millionth to one-hundred millionth of 1 unit of atmospheric strain. This kinds a skinny movie over the polished lens.
9. Completion
An skilled technician inspects purity utilizing an interferometer. Solely lens parts that go the inspection are despatched to be assembled in lenses.
Lenses that make use of fluorite lens parts (as at Might 2021)
For the reason that FL-F300mm, Canon has produced 39 extra lenses that make use of fluorite lens parts. As fluorite lens parts not solely appropriate chromatic aberration but in addition contribute to lowering the dimensions and weight of merchandise,
they’re proactively utilized in giant telephoto lenses.
These lenses are a lot beloved by many photographers who demand excessive picture high quality at tremendous telephoto focal lengths, together with not solely skilled sports activities photographers and
photojournalists, but in addition fans who {photograph} topics reminiscent of wild birds, trains and plane.
RF600mm F4 L IS USM (launched in 2021)