Most people mention that AR coatings, especially on multielement lenses,
allow higher transmission. While this is true, I don't believe it is
the major goal. Bernard Cleyet came close to mentioning the major goal
when he briefly mentioned lens flare.
Lens flare is a gross example of a more subtle problem that grows to a
major problem in lenses with many surfaces... loss of image contrast,
also known as a fogged image.
Light reflected from any len surface(s) and ultimately going out of the
camera is a minor problem. It just means we need a longer exposure to
compensate for the lost of light.
However, light reflected from any lens surface(s) and ultimately going
into the camera and hitting the film or the CCD is light that not part
of the image. Non-image light that hits the film is like background or
noise. It just "fogs the film." The most notable result is loss of
contrast. This can be subtle, or more severe such as a foggy picture,
or distinct such as a visible flare.
I understand that the fantastic zoom lenses the video cameras used by
the major networks for football games, etc. would be totally useless
without antireflection coatings on all the surfaces. These lenses have
so many surfaces that multiple reflections ending up as "fog" would make
the image unusable.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton, OH 45817