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Diffractive Optics in Canon Lenses


Can anyone point me to a clear explanation of how Diffractive Optics work in Canon lenses? I thought I understood the principle of diffraction (from school experiments involving a surface of water, a wave machine and a diffraction grid). The Wikipedia article Diffraction correlates with my understanding nicely, but without once mentioning camera lenses.

Diagrams on Canon websites seem to illustrate a fresnel lens, like a tiny lighthouse projector lens, which is different from a diffraction grid in my book. I notice that Nikon have started producing similar lenses which they designate 'PF' for Phase Fresnel.

I understand the benefits of DO lenses. A DO element's chromatic aberration works opposite to that of a refractive element. While one bends blue (short wavelength) light more than red (long wavelength), the other bends red light more than blue. If you combine the two you can more or less eliminate chromatic aberration. One reason lenses are so big is because if you bend the light a lot you get a lot of CA. But if you have a way to cancel out CA you can design a lens with stronger elements without the CA you would otherwise suffer. Stronger elements make smaller lenses.

One article I found, in What Digital Camera, states as follows:

"Both Canon and Nikon have lenses in the DSLR systems based on Fresnel optics. Such Canon lenses can be identified by a DO (Diffractive Optics) designation and with Nikon look out for the description Phase Fresnel (PF). Nikon’s approach is arguably not based on diffractive optics, but they do use a Fresnel design."

So what is the crucial difference between Canon's DO lenses and Nikon's PF lenses that makes the Canon lens a Diffractive Optics lens while Nikon's isn't? Anyone know?

Incidentally, Fresnel is pronounced 'fru-NEL' because Monsieur Augustin-Jean Fresnel was a Frenchman.

R6mkII, various lenses, speedlites. Also legacy Canons going back to T90 and even A1.


I thought fresnel lenses were diffractive optics.

Here is an article about the new "gapless" lenses: 

Thanks that's an interesting article.

R6mkII, various lenses, speedlites. Also legacy Canons going back to T90 and even A1.
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