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Developing Canon’s DO lenses

Developing Canon’s DO lenses

June 2009

The release of Canon’s EF70-300mm f/4.5-5.6 DO IS USM lens in May 2004 represented a remarkable achievement in the history of camera lens development. With this lightweight and compact lens, Canon became the first company in the world to have successfully incorporated a DO lens element into a photographic zoom lens. CPN reveals the story of how this amazing technology was developed.

The young Canon engineer Takehiro Nakai and his small team of renegade Canon engineers were in a race against time to build a very special mold for a revolutionary new camera lens. If successful, the team would have been responsible for transforming modern photographic lenses. But with a level of precision required in cutting the element mold, and mold after mold being discarded due to almost imperceptibly minute flaws, would the team ever produce the required results?

Refraction

Camera lenses work by ‘bending’ or focusing light. Conventional camera lenses, binoculars and telescopes rely on phenomena known as refraction to do this. Light hitting the lens at an angle alters direction as it moves from air to glass, and again as it leaves the glass into the air. Camera lenses consist of a series of lens elements, each carefully shaped and sized to ‘bend’ incoming light in the right way before it passes through to the next element and finally onto the film or digital camera sensor.

Diffraction

Another means of bending light is known as diffraction; the tendency for light to spread around the edge of an obstruction, or after passing through an opening. Figure 1 shows what happens to light when it passes through a small slit.

 

When light passes through two or more slits set slightly apart (shown in figure 2), the resulting wave patterns combine. This slit construction is known as a diffraction lattice. Wave patterns amplify in the directions where the phases of the light waves spreading from the two slits match. The result is a change in light path direction.

Because diffraction lattices work by blocking light, they are unsuitable for use in photography. Canon engineer Takehiko Nakai and his team of engineers therefore proposed a transparent diffraction lattice which does not obstruct incoming light (see figure 3).

 

Individual lattices or gratings are axe-blade shaped. Light changes direction because as it leaves the lattice it is slightly out of phase with light leaving its neighbouring lattices. By forming the gratings in concentric circles, the lattice becomes capable of focusing diffracted light to a single point.

Single-layer diffraction lattices are very effective at bending light but unfortunately they send light in more than just the desired direction, rendering them useless for photographic purposes. But Nakai and his team had calculated that if two perfectly matching diffraction lenses were constructed and assembled facing each other – a multi-layer diffractive optical, or ‘DO’ element (see figure 4), 100% of incoming light could be diffracted in a single direction.

Super precision

The level of precision required to achieve this is extraordinary. If the lens were the size of a town with a 10km radius, the permissable surface roughness would be about the thickness of a piece of paper. The smallest grating walls would measure just 79cm; the largest, 107cm. Even at this size a shallow scratch could render the whole lens useless.

Diamond cut

In creating the mold for the lens Nakai and his team were using the highest quality diamonds available. Even the direction that each diamond crystal faced was taken into consideration. After setting up the cutting tool and warming up the equipment, the processing room had to be evacuated and the equipment allowed to idle because even the effects of body temperature would affect the mold’s precision.

A successful mold

After months of trial and error, a mold was produced with the sufficiently stringent levels of precision. A specially developed liquid resin was poured into the mold and then cured with the application of UV light. A 3D ultra-high precision micro fabrication technology developed specifically for the purpose was used to assemble the lens and the world’s first practical multi-layered diffractive optical (DO) element was born.

In 2001 Canon released the world’s first camera lens to incorporate the new multi-layered DO element – the EF400mm f/4 DO IS USM: a lens approximately 26% shorter and 36% lighter than any lens of comparable specifications. Easier to hold and more manoeuvrable than conventional lenses, this and the EF70-300mm f/4-5.6 DO IS USM lens can be used without a tripod in ways unimaginable with traditional telephoto lenses.

Nakai and his fellow engineers had produced a lens that would redefine telephoto photography. For his paper on the development Nakai went on to win the grand prize from the Optical Society of Japan’s Optics Design Group. The lens he helped to design won the European TIPA best lens award.

Three-layer DO lens

In a fixed focus lens, such as the EF400mm f/4 DO IS USM, the angle of light (the incident angle) entering the DO lens is more or less fixed. However, in a zoom lens the angle of view varies as the focal length changes. This can result in significant changes to the incident angle. With two-layer type DO lenses changes in the incident angle cause unwanted diffracted light (flare) in the shot, significantly lowering image quality. Canon compensated for this by developing a new three-layer DO lens element (see figure 5).

 

The resulting EF70-300mm f/4.5-5.6 DO IS USM is 28% shorter overall than existing lenses of comparable focal length range. Moreover, there is virtually no residual chromatic aberration; the lens has levels of contrast and esolution comparable to Canon’s professional L series lenses.

Why DO lenses are so effective

One of the difficulties in conventional lens design is dealing with chromatic aberration, or colour blur. Visible light is made up of three components: blue, green and red. They mix in different quantities to create the wide spectrum of colours we can see. Chromatic aberration in conventional lenses results from the fact that these different colours do not all ‘bend’ or refract to the same degree when they pass through a conventional refraction lens. If it’s not corrected chromatic aberration degrades image quality to unacceptable levels (see figure 6).

 

In conventional lenses concave lens elements are introduced to counter chromatic aberration effects, adding weight and length to the lens. However, with a diffraction lattice the order of colour aberration is the opposite to that of the diffraction (see figure 7).

By combining a refraction optical element and a multi-layered diffractive optical element, colour aberrations can be cancelled out. Colour aberration compensation was previously done by combining convex and concave lenses but it can now be done by using only convex lenses. This makes it possible to make the power of each element group weaker, thereby permitting effective correction of other aberrations besides colour. Moreover, by partially changing the period (spacing) of the gratings, it is also possible to achieve an effect identical to that of an aspherical lens, making it possible to compensate for spherical aberration.

Why are DO lenses smaller?

Much of the length and weight in conventional lenses is introduced to compensate for colour aberrations (see figure 9).

 

Shifting the lens elements closer together increases the effect of colour aberration at the film or image plane. By optimising the dispersion of each lens element colour aberration can be exaggerated and ordered in preparation for correction with a DO lens element (see figure 10).

Finally, the DO lens element is inserted in place of the front lens element, completing the colour aberration compensation (see figure 11).