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 sometimes aren't glass at all. They might, for example, be fluorite.

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.

What does this mean for your images? When light passes through a lens, it refracts – that is, it bends. It also breaks up into its constituent colours, just like light passing through a prism. 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.

Chromatic aberration is inherent in glass elements. The problem occurs because the lens is not able to bring all the different colours (wavelengths of light) to the same focus point and, in the worst cases, coloured fringing is seen along some edges. A fluorite lens element has a lower refractive index, which reduces the effect.

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 an 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. 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.

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. 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.

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.

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, combines the characteristics of aspherical and fluorite elements to enable smaller, lighter lenses with improved performance at smaller apertures.