Nanocoating: New and Different!
To more specifically address the "Nano Crystal Coating" type of multicoating, as other answers seem to be either addressing multicoating in general or think nanotechnology coating is just a marketing term.
Nanocoating is actually NOT the same as multicoating, it is very different in design, and affects light in a different way. To start as simple as possible:
- Multicoating is an advancement on the concept of singlecoating, and is designed on the basis of waveform interference.
- Works by "tuning" reflected light in such a way that the reflected
particles waveforms cancel each other out.
- Nanocoating is a much newer concept, intriguingly based on the structure and design of moth eyes (which barely reflect any light at all.)
- Designed to avoid reflection in the first place, and guide light rays into the lens without allowing them to reflect at all.
Multicoating and Waveform Interference
Light exhibits both particle and waveform properties. As such, two photons can interact in such a way as to cancel each other out. This is best demonstrated with illustration, and I'll borrow a wikipedia image for that purpose. Below is an example of a single-coated lens, and how the coating produces reflected photon waveforms that are in opposition to each other (and therefor capable of canceling each other out):
The anti-reflective coating is designed to be exactly as thick as half the wavelength of the frequency of light. Light will reflect at every intersection of material, such as between the air & coating as well as coating & lens. Since the coating is as thick as half the wavelength of light, the reflection from the air/coating interface negatively interferes with the reflection from the coating/lens interface, and the two cancel each other.
Multicoating works the same way, however with multiple layers of coating at different thicknesses. Since the color of light is determined by its wavelength, coating a lens with several layers of exactly half the wavelength of key frequencies of light (such as violet, blue, blue-green, green, yellow-green, yellow, orange, red) will cancel out considerably more light than a simple single-coating will. Single coatings were generally designed in the green to yellow-green band of light, as they tend to be most prevalent in sunlight and daylight. Multicoating is intended to work on a the full spectrum as much as possible.
Deficiencies of Multicoating
The advent of multicoating was a huge breakthrough in terms of lens transmission (the amount of light they allow to pass), reaching levels as high as 99%. Multicoating is not ideal, though. When strong flare and ghosting do occur, they are only capable of entirely filtering out light reflected at the exact wavelengths each layer is designed to filter out. Wavelengths near the intended frequencies will be mitigated, however they will not be entirely canceled. A bright off-axis non-incicent beam of light, such as from the sun in the corner of a frame, can still create large, bright, and very detrimental flare, ghosting, and contrast reduction even on a lens with multicoating.
Additionally, multicoating is simply taking advantage of a property of light to use a negative property of lenses...reflectance...to minimize the impact that reflectance has on image quality. As such, transmission is not ideal, and up to several percent of incident light can be lost for any given wavelength, usually resulting in 1-2% total loss in transmission PER COATED ELEMENT/GROUP. Granted, that is far lower than the 8-10% or more that used to exist with single coating and uncoated lenses, however in complex lenses with many elements, a considerable amount of light can still be lost overall (i.e. a complex 15 group telephoto lens could end up with 15-30% loss in total transmission in the face of strong flare.)
Improvements with Nanocoating
Nanocoating, unlike multicoating, is not a continued evolution of a previous technology...it is indeed an entirely new approach to solving an old problem. Nanocoating is based on the design of moth eyes, which are known in the scientific community to have one of the lowest reflectance indexes of any material. The general design is based on nano-scale roughly dome/spike-like structures intended to guide as much light as possible into the lens, avoiding reflectance entirely whenever possible.
If and when flare or ghosting does occur, since nanocoating is not designed to work on any given wavelength of light but light in totality, the resulting artifacts or loss of contrast is considerably less than with a multicoated lens. In many cases, careful and close scrutiny is required to find small elements of flare and ghosting in photo taken with a nanocoated lens, and when it does exist, it often does not detrimentally affect IQ.
Transmission levels for nanocoating are at least 99.95% PER COATED ELEMENT/GROUP. At a lost of 0.05% or less, the grand total transmission loss for any lens, even complex lenses with many element groups, will remain very low (i.e. a complex 15 group telephoto lens would end up with a total of 0.75% transmission loss.)
(NOTE: The exact nature of light that passes through a nanocoat is not widely publicized, so I can only base my explanation here off of what I have seen and read. I am not claiming 100% accuracy, however I think it is generally accurate enough.)
(I intend to provide an image demonstrating the design of nanocoating, however I need some time to create it.)
