Take a look at the front of any pair of binoculars, refractor telescope, or even most sets of eyeglasses and you might notice that they often have a blue, green, or even purplish tint to them. This is a sign of an Optical Coating.
Optical coatings have vastly improved the quality of optics over the past decades. The reason for their existence is simple: Every time light goes from air to glass it loses as much as 8% of the light due to scattering/reflection. This may not sound like much but when you consider that a basic refractor telescope with an air spaced achromat (lens with two or more elements) and a decent two element eyepiece will have 4 air-to-glass transmissions. So thats ((((100%*.92)*.92)*.92)*.92) = 71.6%. You lose over 1/4 of the telescope’s light gathering power from scattering! That doesn’t include a mirror or prism diagonal which will also lose light! Binoculars have a similar problem, only more so as they have internal prisms that add even more air-to-glass transmissions. (Eyeglasses do not need coatings so much, but they do not cost too much and usually don’t hurt to add).
Hence optical coatings were developed. Coatings do not solve the light scattering problem, but they sure as heck reduce it. A decent coating can reduce the light scattering from 8% loss to around 4-6%. Adding even more layers, or multi-coatings improves the scattering reduction, as does the quality of the coating application. But nothing completely ends scattering.
Optical coatings in lenses are almost always made of Magnesium Flouride (MgFL). They are applied in a vacuum chamber. A single optical coating should have a very subtle blue color when you look at the lens surface at an angle. Multi-Coats will have a green or even purple tint when you examine the surface.
In all, there are several accepted classifications for optical coatings: Coated, fully coated, multi-coated, and fully multi-coated. They are defined as follows:
Coated means that at least one air-to-glass surface in the optical system has an optical coating.
Fully Coated means that all air-to-glass surfaces in the optical system have an optical coating.
Multi-Coated means that at least one air-to-glass surface in the system has an optical multi-coat. The rest should have at least a single optical coating.
Fully Multi-Coated means that all air-to-glass surfaces in the system have mutli-coatings.
There are some variations on this. In Europe optical multi-coats Powdercoating are often referred to as ‘Overcoated’. Adding further to the confusion is that some companies give names to their coatings. Sometimes this is warranted as the coating is somewhat different from standard coatings, more often it is just marketing.
There is a dark side to coatings, the worst of which is the abuse of ‘Ruby Coatings’ on binoculars. Ruby coatings first came out from Steiner Binoculars who are fond of making specialty binocular coatings for specific environments, such as hunting binoculars whose coatings that bring out objects better in foliage, desert coatings for the same purpose in the sandy zones, etc. One coating developed by Steiner had a ruby color. It worked acceptably, but it worked by partially blocking some of the wavelengths of light. The Steiner version did this only a little, but the resulting coating had a problem: It looked cool.
Since the Ruby coatings looked cool, Hollywood and Madison Avenue started using binoculars in ads and movies with the ruby coatings. This turned out to be a bad thing as many low-end binocular producers started churning out ruby-coated binoculars by the container-load. But there was a problem: Ruby Coatings didn’t give you a very good image.
If you look through a Ruby coated binocular and compare it with a normally coated binocular you will see that it looks a little washed out. The greens and blues stand out too much, the reds are faded. Its like looking at a photo that has had some sun bleaching. This is because the red portion of the spectrum is being deflected by the Ruby coatings, which low-end binocular producers made with brighter and brighter red coatings that dropped more and more red light.