Diffraction is an optical phenomenon in which light waves bend as they pass around the edges of an object. In the late 1990s, a team of young Canon engineers came up with the idea of using this phenomenon to counteract chromatic aberration.
A single diffractive optical (DO) element features diffractive gratings – very fine parallel grooves on the surface – which change the direction of light. However, this process generates diffracted light that is not suitable for photographic lenses and
can cause flaring.
Employing multiple DO elements ensures that nearly all light allowed to pass through the DO elements can be used for photography. Canon DO lenses use multiple DO elements whose diffractive gratings are bonded face-to-face. Combining this arrangement with
a refractive convex lens almost completely cancels out chromatic aberration while allowing the lens elements to be placed much closer together within the lens barrel. The result is a high performance EF lens that's significantly shorter and lighter than
typical refractive lenses. In fact, the EF 400mm f/4 DO IS USM is 27% smaller and 31% lighter than a conventional 400mm f/4 lens would be.
Fast becoming a favourite with sports and news photographers, DO lenses are making high performance telephoto shooting easier and more manageable than ever before.