Multi-layer Diffractive Optical Element

Find out about Canon's Multi-layer Diffractive Optical Element technology, which combines the characteristics of aspherical and fluorite elements to enable smaller, lighter lenses with improved performance at smaller apertures.

Having pioneered both aspherical and fluorite lens elements, Canon went on to develop a technology that combines the characteristics of both. Multi-layer Diffractive Optical Element (DO) technology was announced in September 2000, and a prototype lens was shown at the Photokina 2000 exhibition in Cologne. Canon EF series lenses that incorporate DO technology have "DO" in their names, like the EF 70-300mm f/4.5-5.6 DO IS USM, although Canon RF series lenses, like the RF 800mm F11 IS STM, no longer follow this convention, bringing them in line with all other RF lenses, which do not include lens materials in their names.

Diffractive optical elements use a diffraction grating that alters the path of light rays. Diffraction is encountered in ordinary lenses when a small aperture is used. Light rays passing through this aperture are bent very slightly, so that they are no longer travelling in a straight line. This affects the focusing and reduces the resolution of the lens. This diffraction is the reason most lenses give their best performance at an aperture about two stops below maximum, rather than at the smallest apertures.

However, a diffraction grating can be used to introduce corrections rather than create aberrations. Diffraction gratings look a bit like miniature versions of the fresnel lenses used in a lighthouse. They are widely used in spectroscopes and in the optical signal-reading systems of CD and DVD players.

Until 2000, diffractive elements had not been used in camera lenses because there is a tendency for white light to produce superfluous diffracted light as it passes through the grating. This results in flare, which degrades the image quality.

Canon resolved this problem by creating a multi-layer construction made from two single-layer diffractive optical elements with opposing concentric circular diffraction gratings. When incident light enters the element, superfluous diffracted light is not produced and almost all of the light is used for the image. This makes it possible to use a diffractive optical element in a camera lens.

Diagram of a Multi-Layer Diffractive Optical Element from the front and the side, plus Canon's gapless design from the side.

Left: a representation of a Multi-Layer Diffractive Optical Element (viewed from front and side). The diffraction grating is much finer than indicated here – during its manufacture, the grating's height and pitch as well as its positioning requires micron-level precision (one micron equals one-thousandth of a millimetre). Right: Canon's innovation of a gapless dual-layered DO element minimises the flare that can result from the air gap between the two diffraction gratings of the previous design.

Diagram of a DO lens element, showing chromatic aberration with wavelengths coming to different focus points in reverse order.

The difference is that the DO element focuses the wavelengths in reverse order compared to conventional optical elements.

Diagram of a conventional lens, showing chromatic aberration caused by different wavelengths of light coming to a focus at different points.

Chromatic aberration, where light of different wavelengths comes to a focus at different positions on the optical axis, is a characteristic of both conventional glass elements (shown here) and the Multi-layer Diffractive Optical (DO) Element (next picture).

Diagram of a DO lens combined with a conventional lens, showing how the chromatic aberration is cancelled out.

By combining a DO element (left) with a conventional lens element (right), chromatic aberration can be eliminated.

The most significant characteristic of the diffractive optical element is that the positions where the wavelengths combine to form an image are reversed from those of a refractive optical element. By combining a Multi-Layer Diffractive Optical Element and a refractive optical element within the same optical system, chromatic aberration can be corrected even more effectively than with a fluorite element. Also, by adjusting the pitch (spacing) of the diffraction grating, the diffractive optical element makes possible the same optical characteristics as a ground and polished aspherical surface, which effectively corrects for spherical and other aberrations.

Using a DO element also enables a lens to be made much more compact than it would be with a standard telephoto design. The EF 400mm f/4 DO II IS USM, for example, is about 26% shorter and 36% lighter than an equivalent non-DO 400mm f/4 lens.

The Canon EF 400mm f/4 DO IS II USM lens.

The Canon EF 400mm f/4 DO II IS USM lens, a super telephoto lens with Diffractive Optics for a more compact, lightweight, high-performance sports lens.

Cutaway drawing of the Canon EF 400mm f/4 DO IS II USM lens, showing the Diffractive Optical Element.

This cutaway drawing shows how the DO element takes its place in the optics of the lens.

Gapless dual-layered diffractive optical elements

In September 2014, Canon announced the EF 400mm f/4 DO II IS USM. This compact super-telephoto lens debuted a new-generation diffractive optical element that reduces the flare that can sometimes occur in earlier DO lenses.

The original dual-layer DO design sandwiches a layer of air between the two diffraction gratings. This air and the material that makes up the gratings can cause some ring-shaped flaring around bright light sources in the image. Switching to a gapless design, plus the use of a new material for the gratings, reduces the occurrence of flare.

The gapless dual-layered DO is also positioned deeper into the lens and further from the front element in the EF 400mm f/4 DO II IS USM than in the EF 400mm f/4 DO USM. This means that it is less exposed to unwanted light, which can cause flare. Another impact of this change is that the angle of the light reaching the lens is closer to perpendicular, so less light is reflected and the likelihood of flare with backlit subjects is reduced even further.

Cutaway drawing of the optical elements in a super-telephoto lens.

A representation of the optical elements in a super-telephoto lens – beautifully designed, precision engineered, but quite bulky.

Cutaway drawing of a super-telephoto lens to the same scale, showing how the use of a DO element means the lens can be made smaller and lighter.

A lens with a Multi-Layer Diffractive Optical Element can be made smaller and lighter than an equivalent lens manufactured with conventional optical elements.

RF lenses

As mentioned, using a Multi-layer Diffractive Optical Element enables a lens to be made considerably smaller than traditional lens construction. This is especially beneficial with super-telephoto lenses like the RF 600mm F11 IS STM and RF 800mm F11 IS STM, which are designed for use on full-frame mirrorless EOS R System cameras.

The RF 600mm F11 IS STM is constructed with 10 elements in 7 groups, while the RF 800mm F11 IS STM has 11 elements in 8 groups, and both include a Multi-layer Diffractive Optical Element.

At 269.5mm in length when it's extended ready for use, the RF 600mm F11 IS STM is almost 40% shorter than than EF 600mm f/4L IS III USM. Similarly, the RF 800mm F11 IS STM is almost 24% shorter than the EF 800mm f/5.6L IS USM. This reduction in their working length makes the lenses easier to use hand-held, and they feel more balanced on a camera, especially when you're shooting without an optional Battery Grip.

Both RF lenses also feature a collapsible design, meaning they can contract to 199.5mm and 281.8mm in length respectively, making them easier to transport and store. With a bladeless aperture fixed at f/11, the lenses also capture attractively smooth bokeh, while the Image Stabilization helps to deliver sharp images.

Using a fixed aperture reduces the number of variables that need to be considered in the design of the lenses. This makes it possible to optimise the optical construction to make the best use of the DO element and maximise image quality.

Autors Angela Nicholson

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