Multi-layer Diffractive Optical Element

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.

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.

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.

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.