Back when Pentax started hyping their "seven layer multicoating" I asked the Leitz rep about it and why Leica wasn't multi-coating. He replied that they'd been doing it for years, as had other companies, but never thought it was out of the ordinary, certainly not something to build an entire marketing campaign on! He explained that the curvature of the elements in relation to one another has a lot to do with the severity of the reflections so not all lens surfaces required multi-coating, or at least not as many layers. He also said that several of the much heralded Seven Layers served no real optical function. One was applied because it bonded well to the glass, another because it made for a good hard scratch resistant coating on the external elements. The function of some others might be only to seperate two layers from one another, layers that actually had an optical function.
