It is not something most photographers--pros or amateurs--think about, but their work would be much harder without lens coatings. The reason is that lenses work better with coatings because they allow more light to come into the camera for better exposures. Plus, coatings cut down on light flares and image ghosts.
History
Up until the 1950s, camera lenses were not coated. The Germans invented the process, but kept it secret until after World War II. Uncoated lenses let in less light and more lens flare, so photographing images was harder. Exposures had to be longer and precautions had to be taken to prevent lens flare, if at all possible. Contrast also was diminished with uncoated glass. Approximately 7 percent of the light hitting each element of a lens is reflected back. Since modern lenses may have upward of nine elements, this drastically decreases the light reaching the film or charge-coupled device (CCD) digital sensor.
Single Coating
When coating was introduced in the 1950s, it was a single coat. This increased the amount of light coming into the lens and decreased the amount of lens flare and ghost images. This was the standard for both lenses and lens filters until the 1970s. While multi-coating was invented by Katherine Burr in 1938 at General Electric, according to the Glass, Metal and Silicium website, it did not surface for photographers until the Carl Zeiss Oberkochen Company partnered with Asahi Pentax in the 1970s. The two companies created a new multi-coating process that was reasonably inexpensive.
Multi-Coating
Multi-coating lenses increases the amount of light acquired and decreases reflection more than single-coating does. Pentax called its process super multi-coating and the "SMC" label still is attached to most of its lenses. Other camera manufacturers developed their own processes and soon marketed multi-coated lenses.
Differences
You can distinguish between uncoated, single-coated and multi-coated lenses, even if you do not know the year they were made. To do this, take a look at how they reflect light. Uncoated lenses will reflect white light. Single-coated lenses will reflected darker light, usually with an amber and dark blue cast. Multi-coated lenses will appear much darker with a magenta and green cast.
Significance
Since nearly all lenses made for single-lens reflex cameras in the past few decades have multi-coating, chances are you do not even notice you are benefiting from it. Single-use and lower-end cameras often use uncoated lenses to keep costs down, so they gather less light and are susceptible to flares. When using these cameras, or cameras with lenses made before 1972, expect longer exposures and consider shooting to avoid lens flares.
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