How lenses are made
Rod Lawton visits Canon’s Utsunomiya lens factory to find out what goes into a premium-quality lens
Take a look around a lens factory to see how the magic glass is made
It’s not fair, is it? New cameras get all the headlines, while lenses are the poor relation – occasionally exciting but mostly rather expensive add-ons for our cameras. However, while cameras come and go, lenses are for life. Many photographers have favourite optics that they’re still using on their second or third cameras, so what seems like an expensive investment at the time can often prove to be a long-term asset.
The fact is that really good lenses are really quite difficult to make. They demand complex design processes, exotic and experimental glass, and sophisticated processes that are surprisingly reliant on hand-built craftsmanship. Robots are useful, but they have to be taught and programmed by humans first.
And you thought glass was just glass…?
Glass is not the easiest material to work with, but its combination of attributes is unbeatable. It’s transparent to visible light, it’s chemically and thermally stable, and it’s relatively easy to shape.
Glass has an extremely useful property – refraction – that allows optical engineers to bend and focus light. It also has an annoying property, technically called dispersion but known commonly as the prism effect, where white light is split into a spectrum of colours that focus at different points. This is why you get axial and lateral chromatic aberration and why lens designs contain complex combinations of lens elements. Typically one element is used to counteract dispersion created by another.
Glass comes in many different formulations that are still under development to this day, so advances in glass manufacture often yield opportunities for lens makers to design a new type of lens or improve the design, size or operation of an existing lens.
Different types of glass produce different levels of dispersion and refraction. ‘Flint’ glass has a high refractive index (it bends light sharply) but low dispersion (low prism effect), while ‘Crown’ glass offers low refraction and low dispersion.
The dispersion of a glass is quoted as its Abbe number. (Ernst Abbe was an early optical pioneer who worked for Zeiss.) LD (low dispersion), ED (extralow dispersion) and UD (ultralow dispersion) lenses are more expensive but more effective for aberration control.
Fluorite has some terrific properties for lenses, notably the complete removal of residual chromatic aberration and the ability to shorten the total lens length. However, natural fluorite contains too many impurities to be used for lens manufacture, as the large crystals needed can’t be obtained naturally. Canon has the technology to ‘grow’ crystals artificially, however, and this fluorite lens technology has been adopted widely in highperformance L-series telephotos.
It has also used the unique properties of diffraction gratings in its DO (diffractive optic) lenses. Diffraction gratings bend light in the same way as glass lenses, but reverse the usual pattern of dispersion, so in principle they are ideal for correcting chromatic aberration. But diffraction gratings also produce light scatter and flare – both highly undesirable in a lens – so Canon has developed a way of combining two diffractive layers to eliminate this scatter. This has made it possible to produce DO telephoto lenses shorter and lighter than their counterparts.
Canon has also used BR (blue spectrum refractive) lenses to
practically eliminate chromatic aberration in certain fast prime lenses. These are actually not made of glass, but a kind of resin.
Spherical vs aspherical
Modern lenses are made up of a complex configuration of individual lens elements, sometimes ‘cemented’ together in pairs or, occasionally triplets, and often fixed into discreet ‘groups’, including focus groups and zoom groups for lenses with internal zoom. The individual lens elements in a lens fall into two types: spherical or aspherical.
Most lenses are spherical, which means their surface follows the same curvature as a sphere. Some have very shallow curvature, some have strong curvature; some are convex (they bulge outwards), while some are concave (the are dish-shaped); but they all share this same spherical curvature. It’s this that makes it practical to grind and polish all these lenses using automated machinery and in a timely, cost-effective manner.
But some lens designs can’t be achieved using spherical lenses alone. Customer demands for faster, sharper, longer-zoom lenses mean that designers will often have to resort to aspherical lenses to meet the design specifications. This is where it gets a little more complicated…