| Coating | When light enters and exits a lens, approximately 5% of the light is reflected back at each lens-air boundary due to the difference in index of refraction. This not only reduces the amount of light passing through the lens but can also lead to repeating reflections which can cause unwanted flare or ghost images. To prevent this reflection, lenses are processed with a special coating. Basically this is carried out using vacuum vapor deposition to coat the lens with a thin film having a thickness l/4 the wavelength of the light to be affected, with the film made of a substance (such as magnesium fluoride) which has an index of refraction of n, where n is the index of refraction of the lens glass. Instead of a single coating affecting only a single wavelength, however, EF lenses feature a superior multi layer coating (multiple layers of vapor deposited film reducing the reflection rate to 0.2-0.3%) which effectively prevents reflections of all wavelengths in the visible light range. Lens coating is carried out not only to prevent reflections, however. By coating the various lens elements with appropriate substances having different properties, coating plays an important role in providing the overall lens system with optimum color balance characteristics. |
