Thirty three years ago, back when I was ten, I had a rather peculiar hobby: holography. It’s been a while, but I still remember the difference between Leith-Upatnieks and Denisyuk methods. And of course I know the difference between diffraction and refraction!…
…But most people, including photographers, don’t. Vast majority of photographers are absolutely certain that “diffractive optical element” is a fancy name for a good ol’ Fresnel lens (hello Ken Rockwell!).
NB: If you have no idea what I’m talking about, stop reading right now to remain more or less sane.
Note to the optical engineers and quantum physicists out there: I will be trying to explain some crazy ass science in layman’s terms. Pretty similar to how one former US Senator described the Internet as a “series of tubes” and got ridiculed for it… despite the fact that his description was spot on.
You see, Nikola Tesla invented great many things related to electricity, including certain generators and transformers. Your computer’s power adaptor works on a same principle, for example. And it has coils. But can we call it a “Tesla coil“?…
Similar story with Fresnel and his lenses. Augustin-Jean Fresnel was one of the greatest contributors to the theory of wave optics, but he’s best known as an inventor of Fresnel lens. His less known brainchild is “Fresnel diffraction”, which is a generalized theory of scalar diffraction. See the similarities to Tesla and his flock of coils?
Now, pay attention please: Fresnel lens is a refractive optical device, not diffractive.
Refractive elements use their shape to bend the light. Diffractive optics break up incoming waves of light into a large number of waves, then recombine them to form completely new waves. In case of DO or PF elements, we basically have a hologram of a lens instead of the real thing.
…Actually, judging by the optical diagrams provided by Canon and Nikon, we have a hologram of an anti-lens. They use diffractive optics to cancel out chromatic aberrations by introducing holographic counter-aberrations! Damn smart.
Hopefully, my brief explanation will help clear out this widespread misconception. Diffractive optical designs are loosely based on Fresnel’s theories, but they are not “Fresnel lenses”.
PS: Holy guacamole, this is by far the most popular post I ever wrote in English. Lots of monthly visits from all kinds of eye-popping referrers, from NASA to JAXA, from MIT to Lensrentals… Even Nikon’s internal portal seems to be linking here. Goodbye my SEO relevance, I gonna miss ya.
PPS: Drop a comment, don’t be a stranger.