Everyone is familiar with the purpose of a lens - to transmit light to a focal plane, be it film or a digital sensor. What people may not realise is the obstacles that light faces as it passes through each element, and thus the importance of lens coatings.
When light enters a lens, rather than follow a straight path towards the focal plane, many things happen. A small percentage of the light is reflected back at the surface of each element, resulting in a loss in light transmission. This can be as little as 4% though some manufacturers state it could
be up to 10%. As lenses can consist of anything between five and 25 elements, by the time the light has passed through the last element, you can be sure that a large proportion of the light that first entered the lens will have been lost.
Different types of glass also absorb different wavelengths of light (and therefore colours) in varying degrees, which can result in colour inaccuracies on the final images. Lenses tested for colour bias used to be done so by Colour Contribution Index (CCI), a system devised to measure the red, green and blue values of a lens's light transmissions by the International Organisation for Standardisation - the same body who defined speed ratings for film.
Lens coatings were designed to address both of these problems, in addition to suppressing the effects of flare and ghosting which form as a result of light reflections inside a lens. Elements can be coated by a single layer of film optimised for a particular wavelength, or multicoated to help cover a broader range of the spectrum. They work by reflecting a wavelength of light to effectively ‘cancel out' a percentage of the wavelength being reflected by the lens, therefore increasing what light passes through.
These lens coatings are typically thin layers of magnesium fluoride, though different materials are also used depending on their refractive properties. Materials with high and low indexes of refraction are sometimes used in conjunction with each other as lens coatings, much in the way that elements with high and low refractive indexes are used to balance angles of refraction.