LanguagesLenses: Multi-layer Diffractive Optical Element
Having pioneered both aspherical and fluorite elements, Canon went on to develop a technology that combines the characteristics of both. Multi-layer Diffractive Optical Element (DO) was announced in September 2000 – and a prototype lens was shown at the Photokina 2000 exhibition in Cologne.
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 the ray is no longer a straight line. This affects the focusing and reduces the resolution of the lens. 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 are already used in spectroscopes and in the optical signal-reading systems of CD and DVD players.
Until now, diffractive elements have 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 has 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, for the first time, 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.
A diagrammatic representation of the Multi-Layer Diffractive Optical Element. The grating is much finer than indicated here. During the manufacture of the Multi-Layer Diffractive Optical Element’s diffraction grating, the height and pitch of the diffraction grating as well as its positioning requires micron-level precision (1 micron equals 1/1000 mm).
A single diffraction grating (left) creates a lot of superfluous light which degrades the final image. By combining two gratings (right), Canon has overcome this problem.
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 (left top) and the Multi-layer Diffractive Optical (DO) Element (left bottom). However, the DO element focuses the wavelengths in a reverse order to conventional optical elements. By combining a DO element with a conventional element (right), chromatic aberration can be eliminated.
A lens with a Multi-Layer Diffractive Optical Element can be made smaller and lighter than an equivalent lens manufactured with conventional optical elements.
DO lenses
There are currently two DO lenses in the Canon range:
EF400mm f/4 DO IS USM
introduced November 2001
This lens uses a multi-layer diffractive optical (DO) element to correct chromatic aberrations encountered in ordinary lenses at small apertures.
Until now, diffractive elements have not been used in camera lenses because white light can produce superfluous diffracted light as it passes through the grating, resulting in flare. Canon has resolved this problem by creating a multi-layer construction made from two single-layer diffractive optical elements with opposing concentric circular diffraction gratings.
The most significant characteristics of the diffractive optical element is that the positions where the wavelengths combine to form an image (the focal plane) are the reverse of those of a refractive optical element. By combining a multi-layer diffractive optical element with a refractive element, it is possible to correct chromatic aberrations more effectively than before.
Also, by adjusting the pitch (spacing) of the diffraction grating, the diffractive optical element can take in similar characteristics to a ground and polished aspherical surface, which effectively corrects for spherical and other aberrations.
One of the major advantages of the new 400mm DO lens is that it is about 26% shorter and 36% lighter than an equivalent non-DO 400mm f/4 lens. Making the lens more compact and lighter with conventional lens elements causes aberrations, but with the DO element these aberrations can be reduced, allowing a shorter lens to be produced.
- Lens construction: 17 elements in 13 groups (protective glass and drop-in filter included)
- Diagonal angle of view: 6°10'
- Focus adjustment: ring-type USM, inner focusing system, full-time manual focus
- Focusing range: 3.5m to infinity
- Magnification: 0.12x
- Filter size: 52mm rear drop-in type
- Max. Diameter x length, weight: 128x233mm
- Weight: 1940g
EF70-300mm f/4.5-5.6 DO IS USM
introduced May 2004
The EF 70-300mm telephoto zoom features a triple-layed diffractive optical element (DO), enabling Canon to produce a 70-300mm lens less than 10cm long.
DO elements
The EF70-300mm zoom is the second DO lens to be released by Canon. Use of the new three-layer DO lens drastically reduces the overall length and weight of the lens system, increasing the refractive power of each lens element and reducing the spaces between elements. Chromatic aberrations normally associated with increased compactness are corrected by positioning the three-layer type DO lens on the front lens side. This, coupled with the correction of spherical aberration through the use of aspherical surfaces, delivers high image quality and high levels of resolution and contrast, rivalling the performance of Canon’s L-series lenses.
Attractive defocusing
In the 70-300mm lens, appropriate refractive and DO elements are combined to realise a similar high performance. The lens incorporates a near circular aperture diaphragm for beautifully pleasing background blur (bokeh).
- Lens construction: 18 elements in 12 groups
- Diagonal angle of view: 34° to 8°15’
- AF drive system: Ring USM
- Manual focusing: Enabled with focus mode switch and focusing ring (full time manual focusing)
- Focusing range: 1.4m to infinity
- Maximum magnification: 0.06x (wide); 0.19x (tele)
- Minimum aperture: f/32 to f/38
- Aperture blades: 6 (circular type)
- Filter diameter: 58mm
- Diameter x length: 82.4x99.9mm
- Weight: 720g
- Lens hood/lens cap: ET-65B/E-58U
- Lens case: LP1116
- Lens Extender EF: not compatible
