Stay at home
- Aug 16, 2012
I am not nitpicking but just want to summarise my attempts to understand the lens technologies. The Fresnel lenses used in lighthouses are a different technology from the DO of Canon and the Phase Fresnel of Nikon. The lighthouse lenses are pure refractive lenses - Fresnel compacted a very thick lens into a thin one made of concentric rings with the same curvature as the thick one, and focuses light by bending it like the thick lens. The DO and Phase Fresnel are pure diffraction. The diffraction lens had its forerunner in the Fresnel Zone Plate, which has concentric rings of light transmitting and absorbing materials that diffract and focus light with about 10% efficiency. Then someone had the bright idea of replacing the absorbing material with rings of transmitting material that retarded the phase by pi, and diffracted the light to the same point and doubled the light. Then in 1961, Miyamamoto invented the Phase Fresnel lens which looks like a micro version of the original lighthouse lens but diffracts with phases differing by modulo 2pi. It acts like an analog zone plate but with near 100% efficiency.I have taken photos with the R5 and RF 800mm f11 lens, and it can AF just fine with it.
When I look at the front DO element, I'm amazed that at first glance it looks like a normal smooth lens. Only when I look closely at it with a strong light reflected off its surface can I notice the very slight edges of the tiny DO "rings" in it. It's amazing that the DO technology of the huge original lighthouse lenses can be shrunk and mass produced to give such good IQ. I know that it can't possibly have the same high quality contrast as a normal high quality lens due to those edges, but the overall IQ is amazing considering what the old lighthouse lenses looked like!