Fluorite, aspherical, UD and BR lenses

Photographers sometimes like to refer to lenses loosely as "glass", but of course they're much more complex than just pieces of glass, and the optical elements in modern lenses can include materials such as fluorite and plastics as well as different varieties of glass. All these materials, each with unique properties, are used to improve optical performance, from reducing aberrations to enhancing sharpness and contrast.

In this guide, we'll explore some of the materials and technologies used in lens elements and how they contribute to improving image quality.

What is chromatic aberration?
Fluorite lenses
Spherical lenses vs aspherical lenses
Manufacturing aspherical lens elements
Ultra-low Dispersion glass
Blue Spectrum Refractive (BR) elements

When light passes through a lens, it refracts – that is, it bends. What's more, different colours (wavelengths of light) are refracted to different degrees, with the result that it also breaks up into its constituent colours, just like light passing through a prism.

This phenomenon is called chromatic aberration. It means the lens is not able to bring all the different colours to the same focus point and produces a fuzzy image. In the worst cases, coloured fringing is seen along some edges.

Chromatic aberration is inherent in glass elements because of the physical properties of glass, but other materials can alleviate the problem to some extent. The lower the refractive index of the lens material, the less the light bends and the sharper the image. Similarly, the lower the dispersion ratio, the less the light is broken up, which makes it easier to correct chromatic aberration.

It's worth noting that in addition to optical solutions, Canon has developed advanced ways to tackle chromatic aberration in images in post-production. Canon's Neural Network technology uses artificial intelligence to analyse images, detect chromatic aberration, and correct it intelligently, resulting in clean, crisp images with true-to-life colours.

Fluorite is a naturally occurring crystal with three special properties that make it eminently suitable for use in lenses − it transmits infrared and ultraviolet light well, has a very low refractive index, and has low dispersion. This means fluorite lens elements greatly reduce chromatic aberration, compared to glass lenses.

Even back in the 19th century, natural fluorite crystals were used in microscope lenses, but in nature, fluorite grows in very small crystals and it's not suitable for use in photographic lenses. To overcome this problem, Canon grows its own synthetic fluorite crystals in large enough quantities to create photographic lenses from them.

The next stage is to grind the fluorite into lenses − another challenge, since fluorite is difficult to grind. However, the engineers at Canon developed a new grinding technique to ensure flawless fluorite lens elements. The downside is that it takes four times longer to grind a fluorite element than a glass element − one of the reasons for the increased cost of a Canon L-series lens. The results, though, are lenses that all but eliminate chromatic aberration, resulting in sharper images since the light is recorded as a point rather than a blur of colours.

The first Canon lens to contain a fluorite element was the FL-F 300mm f/5.6, produced in 1969.

In the early days, all lenses were spherical. They are the easiest lens shape to make, but are not best suited to rendering a sharp image because they cannot make parallel rays of light converge at the same point. This causes a problem called spherical aberration. Lens designers discovered that an aspherical lens shape would eliminate this type of aberration, because the curvature of the lens could be used to converge the light rays to a single point. But knowing the theory is one thing − putting it into practice is quite another.

The degree of asphericity is so minor that special manufacturing processes were created to stay within the 0.1-micron tolerance required. Measuring the curvature requires even greater accuracy. It was not until 1971 that the first SLR camera lens with an aspherical lens element was produced, the Canon FD55mm f/1.2AL. But it was not perfect. In fact, it took another two years before manufacturing techniques reached the levels required to achieve really large gains in image sharpness.

Today, aspherical lens elements are so precisely ground and polished that if the degree of asphericity is even 0.02 micron (1/50,000th of a millimetre) away from ideal, the element is rejected.

Aspherical lens elements help to compensate for distortion in wide-angle lenses, and compensate for (or even eliminate) spherical aberrations in lenses with a large maximum aperture. They also enable Canon to produce more compact lenses than was previously possible using only spherical lens elements. This is because traditional lens designs often involved complex arrangements of multiple lens elements to minimise aberrations, but a single aspherical lens can do the same job, resulting in a lighter, more compact lens with superior sharpness.

Ground aspherical lens elements are individually ground and polished to an extremely high level of precision. The process is suitable for different types of glass and can be used to produce aspherical lens elements with large diameters relative to spherical lenses.

Grinding and polishing an aspherical lens element is a lengthy and expensive process, but manufacturing developments now also make it possible for aspherical lenses to be moulded. Plastic moulded (PMo) aspherical lens elements are formed by injecting optical-grade resin into an aspherical surface mould, with coatings then applied to finish the lens. These lens elements have the advantage of light weight, can be mass produced in larger quantities at lower cost, and have made it possible to greatly improve the image quality of entry-level lenses.

Large-diameter glass-moulded (GMo) aspherical lenses are manufactured using various types of optical glass softened by high temperatures and then shaped in an aspherical metal mould. Naturally, the moulds need to be made very precisely to ensure that the molten glass is exactly the right shape. They must also take into account the change in the elements' dimensions once the glass has cooled and been polished.

Glass-moulding allows for manufacturing in quantity, and the resulting lens elements retain the scratch- and heat-resistant properties of glass. Although manufacturing them is still a precision process, moulded elements are less expensive to produce than ground elements, making their use in consumer lenses feasible.

In 1990, Canon developed a fourth technology to produce "replica" aspherical lenses, using a resin to form an aspherical lens surface layer on a spherical lens element. Optical resin is added on a spherical glass lens, pressed into shape by an aspherical surface press mould, and then hardened by ultraviolet light. This process can be used with different glass materials and sizes of glass base, offering high design flexibility. In addition to being cost-effective, replica aspherical lens elements are also lighter than their ground counterparts.

As the only manufacturer using four different aspherical lens production technologies, Canon is able to cater to diverse needs by choosing the most appropriate of these technologies for each lens element required.

The emergence of UD (ultra-low dispersion) glass and Super-UD glass came after Canon had successfully incorporated fluorite into some of its lenses. Using optical glass rather than fluorite to correct chromatic aberrations is more cost-effective, so Canon directed its research into high-performance lenses manufactured from optical glass. Over the years, Canon has used more than 100 different types of glass in its lenses, each with slightly different properties.

UD glass is similar to fluorite in that it has a low refractive index and low dispersion. Although it is not quite as good as fluorite, its performance is significantly better than ordinary optical glass. So by using UD glass Canon has been able to manufacture a range of lenses with superior performance and at a lower cost than before.

In several L-series lenses, both UD glass and fluorite lens elements have been combined to produce optimum results. The technology is suitable for different kinds of lenses, from wide-angle to super-telephoto.

Blue (short wavelength) light is particularly troublesome for lens engineers because it's hard to correct its path through a lens element in the same way as longer-wavelength green and red light, which means it can cause blue fringing.

However, in August 2015, Canon introduced the EF 35mm f/1.4L II USM, the first lens to feature a Blue Spectrum Refractive (BR) element. The BR element uses a new organic optical element that has different dispersion characteristics from standard elements. It is sandwiched between concave and convex glass lenses, to control the path of blue light and minimise chromatic aberration.

Canon continues to develop new optical materials to further expand the possibilities of lens design and manufacture. Canon's Multi-layer Diffractive Optical Element technology, for example, utilises an optical phenomenon to emulate the characteristics of aspherical and fluorite elements, enabling smaller, lighter telephoto lenses with improved performance at smaller apertures.