Demystifying Diffractive Optics
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.
– Canon calls this technology “Diffractive optics“. Nikon calls it “Phase Fresnel“. How the heck it’s not a Fresnel if even Nikon calls it Fresnel?!
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.
Ok, so what? If DO lenses like the one Canon produces…what’s the benefit to us as photographers? Chromatic aberrations aside, does it render a sharper image? I would say no. You might get less aberration but at what expense? Like…finish the discussion dude. You brought us this far and gave an explanation but now finish and make it RELATIVE to your audience.
So what? I’ve described what’s wrong with a widespread factoid and suggested using facts instead. You can see concentric patterns in many diffractive elements, but that’s not a Fresnel lens. You can see a stick in many cars that have an automatic transmission, but that doesn’t mean that their owners are “driving stick”.
DO lenses are generally smaller, lighter and cheaper (sometimes latter is not the case) than their all-refractive counterparts with similar optical qualities – no less, no more.
PS: Relative discussions with oneself are one of the first signs of schizophrenia.
Like I said, finish. I believe and trust what you are saying is true. Now what the hell do I do with this info other than chalk it up as general knowledge? Are you suggesting that certain DO lenses are worth looking into and if so what do you suggest?
Again, great explanation now take us home!
Imaginary scenario: you open a camera or lens review, for example, on DPReview, look at the table of contents and… there’s no “Conclusion”! I think your first feeling (other than a cognitive dissonance) would be panic. Just a thought.
All DO lenses are worth looking into, especially the long ones like Canon’s 400/4 DO. But with zooms such as 70-300 (if you’re on a market for one) does being 4 cm shorter justify 2x price increase? I don’t think so, unless you’re packing for the space travel. But 70-300 DO is way better than non-DO version when it comes to fringing and flare control. It also focuses quite a bit faster. Again, does it justify the 2x price difference? This is for you to decide.
As always, I recommend renting both lenses in question and shooting them side by side in your typical photographic environment, using your typical kind of subjects and backgrounds. Then you can draw your own conclusions.