Anti-reflection lens coating
1935 (Smakula patent at Zeiss)
A film of magnesium fluoride a quarter of a wavelength thick stops glass surfaces reflecting light. Lenses with many elements became bright and clear instead of hazy.
How it began
Each air-to-glass surface reflects about 4% of the light, so a lens with eight surfaces loses about a third of it, and the stray reflections fog the image as flare. In 1935 Alexander Smakula at Zeiss patented a way to coat lenses with a thin evaporated layer. The German military kept it secret for some years.
How it works
Light reflected from the top of the coating and from the glass beneath it travel distances that differ by half a wavelength, so the two reflections cancel. Magnesium fluoride has the right refractive index to make that cancellation nearly complete for green light.
Why it mattered
Coating made complex lenses practical: more elements meant better correction without a haze of flare. It also improved contrast in colour photography, which was just beginning. Coated lenses became normal after the Second World War.
Limits and what replaced it
One layer works best for one colour. Multi-layer coatings from the 1970s and nanostructured coatings in the 2000s reduced reflections across the whole spectrum.
Today
Every modern lens is multi-coated; without coatings a zoom with twenty elements would lose most of its contrast to flare.