Sito italiano sulla natura e fotografia naturalistica Copyright   ©2001 ARDEA  riproduzione vietata, tutti i diritti della legge sui diritti d'autore

Available in November 2001 will be the innovative EF 400mm f4 DO IS USM lens. This uses a multi-layer diffractive optical (DO) element to correct chromatic aberrations.

Diffractive optical elements use a diffraction grating to alter the path of light rays. Diffraction is encounter in ordinary lenses at small apertures. Light rays passing through the aperture a bent very slightly. This affects the focusing and reduces the resolution of the lens. It is the reason most lenses give their best performance at apertures a couple of stops below the maximum, rather than at the minimum aperture.

However, a diffraction grating can be used to introduce corrections, rather than create aberrations. They have been used for some time in spectroscopes and 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.

The most significant characteristic 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 on 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 f4 lens. Lenses can be made smaller and lighter by reducing the distance between the elements, and increasing the refractive properties of the front and rear lens groups. However, with conventional lens elements, this increases chromatic aberrations. Using the DO element, these aberrations can be reduced.

The lens also incorporates Canon's image stabilization (IS) system to help reduce the effects of camera shake at slower shutter speeds. There is a benefit of around two stops so, for example, you can shoot at 1/30 second with IS with similar results to shooting at 1/125 second without IS.

 It should be available early November 2001.

Specification

Focal length: 400mm

Aperture range: f4 to f32

Aperture blades: 8

Construction: 17 elements in 13 groups (including multi-layer diffractive optical element)

Filter mount: 52mm drop-in

Minimum focus: 3.5 metres

AF drive: ring USM

Manual focusing: enabled with focus mode switch and focusing ring

Extenders: compatible

Extension tubes: compatible with EF12/EF25