[COLOR=darkgreen][B]No.9: [/B][/COLOR][COLOR=black]Anti-reflective lens coatings and why they are needed.[/COLOR]

Most if us must surely have noticed that camera lenses usually have a pretty colour about them from certain angles, though the transmissive quality of the lens is, of course, clear. What you are noticing is the effect of [B][COLOR=darkgreen]ant-reflective[/COLOR][/B] lens [COLOR=darkgreen][B]coatings[/B][/COLOR] applied to the lens surfaces. I say 'surfaces' as most camera lenses are compound, employing several lens elements. Each surface of each element is usually coated with a microscopic layer of clear material of a selected refractive index. The most common material for the job is magnesium fluoride. You can also see the effect of lens coating on spectacles; the surface of the lens may take on a greenish, purple or amber sheen. Un-coated lenses will not exhibit any colouring.

So why are lens coatings used? Approximately four percent of the light that passes through a lens surface is reflected rather than refracted. This extraneous reflected light is a problem because it can scatter around inside the lens and other lens elements in the lens structure and form unwanted and distracting artifacts in images, like flare and ghosting. It also reduces the contrast of the image.

Because light exhibits a wave-like property, by directing the reflected light back on itself the phenomenon of [COLOR=darkgreen][B]destructive interference[/B][/COLOR], where waves of similar wavelength meet out of phase, the waves cancel each other out. Lens coatings enable this to happen for a given wavelength of light.

Anti-reflective coatings for camera lenses were used in the early 1930s and were a German military secret during the second World War. More recently, multiple coating layers have been used to improve efficiency by reducing reflections in more visible wavelengths. Lens coating has marketing value, with various lens makers using terms like T* (Carl Zeiss), Electron Beam Coating (Fujinon EBC), and Super Multi Coating (Pentax SMC).

The most common method for applying lens coatings is through vacuum deposition. The coating material is vapourised in a vacuum and it settles on the surface of the lens to form a sub-micron layer. This is not a cheap process and lenses with multi-layer coatings tend to be more expensive.

[B][COLOR=darkgreen]Tomorrow:[/COLOR][/B] The focal plane shutter.

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