Today's photographic lenses are made of highly complex materials. Lens elements can be made from inorganic substances like mineral glasses or from organic mixtures like highly transparent plastics.
Photographic lenses are predominantly made of optical glass. It is highly transparent to the visible spectrum of light and has refractive properties suitable for image formation. It is very different in composition compared to window and other functional glass. Raw materials used for optical glass must be particularly pure to avoid self-fluorescence of the lens. Historically, optical glass has gone through several stages in its development.
Early optical glasses: Good-quality optical glass was first produced around 1790 in France by an optician named Pierre-Louis Guinand who stirred glass in melting pots. For over 90 years, only two types of optical glasses were available, crown and flint. Crown glass was produced from a melt of the oxides of silicon, aluminum, calcium, lead, potassium and sodium. Flint glass, which is more refractive, was produced by a higher proportion of lead oxide in the melt. Unfortunately, the higher refractive index of flint glasses came with a relatively high level of dispersion and astigmatism. Both aberrations could not be sufficiently corrected at that time.
First significant improvements: It was around 1880 in Germany when physicist Ernst Abbe and chemist Otto Schott discovered new recipes for the production of optical glasses with some advanced optical properties. Their new glasses used compounds of barium, boron, phosphorus and fluorine to achieve glass types with high refractive indices but with low levels of dispersion. This was an important step to allow the production of anastigmatic lenses.
Further developments: An even further improvement was made in the 1930s when glasses were produced from materials including oxides of rare earths such as lanthanum, tantalum and thorium but no silica. These new recipes allowed the production of even higher refractive index glasses with relative low dispersion.
Hazardous materials: Unfortunately, some of the ingredients used to produce optical glasses of the 20th century were potentially hazardous. Some photographic lenses that have been produced from the 1940s through the 1970s are measurably radioactive. The main source of radioactivity is thorium oxide that was used as an ingredient for the production of the glass used in the lens. Today, thorium is no longer used as a component for optical glass production because of its radioactive nature. Similarly, the oxides of beryllium, mercury and tellurium are no longer used because of their highly toxic properties.
The diagram shows eight very popular glass types and their material composition. With all these raw materials available today and with all the combinations of mixing ratios, lens designers can often choose from several hundred glass types that glass manufacturers typically offer.
The table includes a number of glasses produced by most optical manufacturers. The reason why the short names (PSK, LLF, etc.) do not always match the letters of the English glass names is because the abbreviations originate from the German names.
The leading letters in the glass names indicate what dominant chemical element is used in the glass type: F (Fluorine), P (Phosphorus), B (Boron), BA (Barium), and LA (Lanthanum). The last letters in the names typically indicate whether the glass is a crown (K from German 'Kron'), a crownflint (KF from German 'Kronflint') or a flint (F) glass. Another element in the names of optical glasses describes the density of crown and flint glasses. Some flint glasses with different densities are named LLF (very light flint / German: 'Doppelleichtflint'), LF (light flint / German 'Leichtflint'), and SF (dense flint / German 'Schwerflint'). Similarly, crown glasses with different densities are named SK (dense crown / German 'Schwerkron') and SSK (very dense crown / German 'Sehr schwere Krone' or 'Schwerstkron').
Plastic is an umbrella term for a wide variety of synthetic materials that primarily consist of polymers. Some of these polymers are perfectly transparent to the visible spectrum of light, and thus are suitable materials for optical applications. In order to obtain high optical performance, very pure raw materials must be used in combination with optimized processing techniques.
The production of plastic material starts with the creation of smaller chemical structures called monomers. These monomer units consist of different compounds of carbon, hydrogen, oxygen, and sometimes additional atoms. The monomer substance is then subject to polymerization through which macromolecules in the shape of longer chains are formed. These chains can also form additional connections, creating a solid polymer.
Polymers have significantly lower melting points compared to glasses. Plastic materials with smaller polymer chains usually have lower melting points than ones with long polymer chains. Depending on the type of polymer, the melting points of some common products range somewhere between ~100°C and ~250°C. By combining these temperatures with pressure, polymer lenses can be brought into the desired shapes. Glass, by contrast, requires temperatures between 1.000°C and 1.700°C to melt, depending on the type.
Here is a list of four common polymer materials used in optical applications:
The physical and chemical properties of polymers are very different from those of mineral glasses. Lenses made of glass are generally harder, more resistant against high temperatures, and more durable than plastic lenses. Polymer, on the other hand, is lighter and can be used in completely different manufacturing processes. While traditional glass lens elements are made by grinding and polishing the surfaces into their final shapes, polymer lenses are typically made by injection molding, compression molding or casting. These are especially cost-effective manufacturing technologies for the production of aspheric lens elements such as the 'seagull' lens elements inside the Canon RF 28mm F2.8 STM lens. A slightly different approach is used in the production of Blue Spectrum Refractive (BR) lens elements as used in the Canon RF 85mm F1.2L USM lens. The BR lens is formed by injecting molten polymer material between two glass lens elements, fusing all elements together into a triplet lens.
One disadvantage of polymer lenses is the limited range of products. While manufacturers of optical glasses offer several hundreds of different optical materials, the choice for polymers is currently limited to less than 10 commercially available different plastic materials of optical quality.
Looking at glass lenses and plastic lenses from an optical point of view, there are two main aspects in which the many types of optical materials differ from each other: Their index of refraction and their dispersion.
This is the main optical phenomenon that allows photographic lenses to redirect light and to form an image.
Refraction is the change in direction of a light ray when it passes from one transparent substance into another. To be precise, light is refracted when it passes the interface between two transparent substances (for example air and glass) at an angle other than 90 degrees and when these substances have different refractive indices. If these conditions are met, the incident ray of light changes direction. The reason therefore is a change in speed as light travels slower in denser materials, and faster in lighter materials. Here is a summary of how the two factors influence rays of light:
The surface normal is an important guidance to determine the direction in which light bends. As a general rule, a ray of light is always bent more towards the normal in the denser, more refractive material.
Snell's Law is a formula used to quantize the change of direction that a ray of light undergoes. It allows to calculate the exact angle of refraction for a given angle of incidence, and is therefore one of the most basic principles that lens designers have to know.
The law states that, for a given pair of substances, the ratio of the sines of angle of incidence (𝜃1) and angle of refraction (𝜃2) is either equal to the ratio of the phase velocities v1⁄v2 in the two substances, or equivalently, to the ratio of the refractive indices n2⁄n1.
The table shows some refractive indices of some very common transparent materials, measured with a reference wavelength of 589.29 nm (yellow light). Note that air isn't exactly 1 but for reasons of simplification is often specified in diagrams as n = 1.0.
| Substance | Refractive Index |
|---|---|
| Vacuum | 1 (by definition) |
| Air | 1.00027717 |
| Water | 1.33 |
| Fluorite | 1.44 |
| Fused Silica | 1.46 |
| Window Glass | 1.52 |
| Crown Glass | 1.50–1.76 |
| Flint Glass | 1.60–1.92 |
| Diamond | 2.42 |
This is the other major property of optical glass, and the amount of dispersion is what differentiates hundreds of glass types.
In optics, dispersion is a phenomenon where rays of light are refracted (bent) at different angles depending on their wavelength (color). This can best be observed when white light is passed into a prism consisting of glass with a refractive index of roughly 1.6. The beam of light spreads out into a complete spectrum of light ranging from red colors to blue and ultimately violet colors. This demonstrates two things: Firstly, white light is a composite of all rainbow colors, although there are some exceptions to this rule. Secondly, the prism also shows that longer wavelengths (red light) are refracted less than shorter wavelengths (of blue and violet light). Therefore, every distinctive wavelength has its own deviation angle. If the smallest deviation angle of red light is subtracted from the largest deviation angle of violet light the result describes the spread angle.
The reason for white light to disperse when passed through a prism is that transparent substances do not only have one single index of refraction. In reality, optical materials have a variable refractive index depending on the wavelength (color) of light that passes through. The index of refraction that is specified for a particular optical glass is usually valid for a reference wavelength of yellow light (rd. 590 nm) only. Therefore, a glass prism with a refractive index of 1.6 realistically has a range of indices from rd. 1.58 (red light) to rd. 1.62 (violet light). This range continues for the invisible wavelengths of light with an even lower index for infrared and even higher index for ultraviolet light. For that reason, each wavelength gets redirected into a slightly different direction, creating a full spectrum of colors.
The diagram shows the dispersion curves of seven optical glass types. All curves increase towards the left, indicating higher refractive indices at the shorter end of wavelengths. Dispersion curves rise more steeply upwards for flint glasses than for crown glasses, which means that flint glass is more dispersive than crown glass.
The diagram compares two very popular glass types, BK7 and SF11. The dispersion of an optical material can also be described with three refractive indices that are measured for three well-defined wavelengths of light, λF, λd and λC, which are the wavelengths at the Fraunhofer F, d and C spectral lines. The resulting indices are referred to as nF, nd and nC. Low-dispersion glasses have three index values that are closer together whereas high-dispersion glasses have three index values that are further apart.
The emission spectrum of a chemical element can be very different from the solar spectrum of light. Whereas sunlight has a relatively even spread of light intensities throughout the visible wavelengths, some gases can be excited so that they show an emission spectrum with only a few thin lines of certain wavelengths. This characteristic of gases is used in optical research to determine optical properties of glasses. The table shows ten very common elements that can be excited to emit very clear patterns of wavelengths called Fraunhofer spectral lines. The F, d and C lines are used to determine the refractive indices of glass at these different wavelengths.
The Abbe number is a quantization of the amount of dispersion an optical glass shows, almost like a 'dispersion index', although this term isn't used. When the refractive indices of optical glass have been measured for the three wavelengths of the Fraunhofer F, d and C lines, the Abbe number can be calculated using this formula:
vd = nd - 1⁄nF - nC
Glasses with a relatively low Abbe number of less than 50 show strong dispersion and these are called flint glasses. Glasses with a higher Abbe number show relatively low dispersion and these are called crown glasses. Flint glasses typically have relatively high refractive indices (bend light strongly), whereas crown glasses have lower refractive power.
This chart brings both main properties of optical materials together and combines refractive index and abbe number in one visualization.
An Abbe diagram is a very popular visualization of the most common optical glasses and polymers. Each type of optical material is represented by a colored dot. The horizontal coordinates are the Abbe number that increases from right to left, indicating that the dispersion increases from left to right. The vertical coordinates are the refractive index, and a higher number indicates a stronger refraction.
Glasses on the top right are highly refractive but also show significant dispersion. Glasses on the lower left are low refractive index with low dispersion. It is interesting to see that there is no option available for an optical glass with very high refractive index and very low dispersion. Polymers are found in the area below mineral glasses.
While refraction is an effect that is absolutely necessary in optical systems, dispersion is the reason why optical systems exhibit chromatic aberrations that challenge optical designers until today. As dispersion can never be canceled out completely, it is the difficult task of experienced optical designers to combine various glass types in an optical system so that dispersion can be minimized to an acceptable level.
Fluorite is an extremely special transparent material with some outstanding optical properties. It has a very low refractive index of roughly 1.44 and almost no dispersion with an Abbe number of up to 95 (depending on the manufacturer). For this reason, fluorite lenses can be used to reduce color defects (chromatic aberration) that normally occur with lens elements from traditional glass materials.
Chemically, fluorite glass consists of the elements fluorite (F) and calcium (Ca), forming a stable halogenide mineral (CaF2). This mineral exists both in natural and synthetic forms:
Although being a superior optical material, fluorite lenses are very challenging to produce. Fluorite lens elements cannot be molded into a rough shape like traditional glass lens elements but instead have to be sawn from a single large crystal. After that, grinding the lens usually takes four times longer compared to regular glass elements. Being a crystal structure, fluorite lenses tend to chip and can easily be scratched. This makes production and handling of fluorite lenses rather difficult. The advanced processing techniques required to polish fluorite crystals into lens elements and greater production time is what makes fluorite lenses way more expensive than those from optical glasses.
In the 1960s, Canon made groundbreaking advances in the mass-production of synthetically grown fluorite crystals, and became the only manufacturer to incorporate fluorite in SLR lenses at that time. The Canon FL-F 300mm F5.6 S.S.C. FLUORITE – released in 1969 – was the first Canon lens to employ one lens element made from synthetic fluorite. As fluorite lens elements are rather susceptible to heat, Canon often protects their fluorite lenses via a layer of their white heat-shield coating to ensure outstanding optical performance even in hot sunlight.
The process of making synthetic fluorite starts with natural fluorite ore as the raw material. The natural mineral is crushed into a coarse powder. In a first refinement step, impurities are extracted from the raw material. For the actual monocrystal production, the purified powder is transferred into a crucible that is put inside a specialized crystal-growing furnace where it is heated to 1.400 °C. The crucible is then removed very slowly from the heat source by lowering it down. This causes crystallization to occur starting at the bottom of the crucible. Crystallization removes even the last impurities and creates a fluorite ingot of extremely high purity. This artificially grown structure is then slowly cooled down over a period of several weeks to prevent it from cracking. Once cooled down, it is cut into discs and is forwarded to rough processing, grinding and polishing.
Soon after the advent of fluorite lenses, Canon began researching high-performance optical materials with properties similar to fluorite (low refractive index, almost no dispersion) but without the disadvantages associated with complex production, difficult handling and the increased price. Their research was rewarded in the second half of the 1970s when Canon introduced a new material they called ultra-low dispersion (UD) glass. The newly developed UD glass was an optical glass produced from traditional oxide ingredients but with specialized formulas to outperform previously known glass types. Although the newly discovered UD glass wasn't quite as good as fluorite, it represented a significant improvement in terms of cost-efficiency. It allowed Canon to produce a range of photographic lenses with superior optical performance but at lower cost than fluorite. In some L-lenses, lens elements from UD glass and from fluorite have been combined to achieve even greater color accuracy. Popular Canon lenses that use UD glass are the EF 70-200mm F2.8 L IS II USM and the EF 70-200mm F4 L IS USM
In the early 1990s, Canon has introduced a further development of the first ultra-low dispersion glass. The new optical glass was called Super-UD glass. Its production was again based on traditional oxides and processing techniques but the optimized Super-UD glass composition resulted in low levels of refraction and dispersion very close to fluorite but at significantly lower cost. There is a large number of Canon lenses that has one or two Super-UD glass elements embedded, and it is still used in the newest generations of Canon RF lenses.