The refractive index of light changes slightly according to the wavelength. As light passes through a lens, this difference in refractive index results in chromatic aberrations and reduced image quality. One way to correct chromatic aberrations is to combine convex and concave lenses. In the case of telephoto and zoom lenses, in particular, the large number of lens elements needed to correct chromatic aberrations results in larger lens sizes. Canon developed the world's first Diffractive Optical Element (DO lens) for cameras, solving the problem of chromatic aberration and, at the same time, achieving smaller lens sizes.

The development of the DO lens was based on the principle that chromatic aberrations occur in opposite directions in diffractive and refractive optical elements. In other words, it is theoretically possible to eliminate chromatic aberrations in lenses by combining diffractive and refractive optical elements. Since diffractive optical elements have only one diffraction grating, which causes light to branch in unnecessary directions, resulting in such problems as flare, it was unsuitable for use as a camera lens. Canon developed a unique two-layer DO lens by precisely placing two optimized diffractive optical elements a few micrometers apart to create a multi-layer grating. The two-layer DO lens, which Canon incorporated into its EF400mm f/4 DO IS USM interchangeable SLR camera lens, makes possible significant reductions in the size of telephoto lenses, and, at the same time, contributes to greatly improved imaging performance.