EF 400mm f4 DO IS USM
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 f4 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.
Focal length and maximum aperture: 400mm 1:4