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QUESTION: Why does the
reflection from my camera lenses have a different appearance than the
reflection from my car windows or other such glass
surfaces?
ANSWER: The reflection from good photographic lenses should look
different than the reflection from an ordinary piece of glass for a very
good reason. Glass is used as a transparent material in windows because it
is very clear, resistant to the elements, hard, and relatively inexpensive
to manufacture. The glass used in photographic lenses is very clear,
resistant to the elements, hard, but relatively expensive to manufacture.
Before the use of computer control manufacturing methods the glass used
for photographic lenses was more expensive to make because it required
more manufacturing steps and the steps required much closer supervision
than the production of ordinary glass. The use of computers has reduced
the cost of making all types of glasses but photographic grade glass is
still more expensive to produce than fine crystal glass. Once the glass is
made it still has to be shaped into the curved pieces that go into
creating a photographic lens. Each curved piece of glass in a lens is
referred to as an element.
The reason the lens reflection has a
different appearance is because all the lens elements undergo a coating
process before they are placed into the lens. This coating process puts an
extremely thin, hard, transparent coating on each glass surface of the
lens. The reason for the transparent coating or layer is to enable the
lens to transmit more light.
A ray of light encounters two surfaces as
it passes through a lens element. The light enters at the first surface
and leaves at the second surface. The light divides into two parts as it
strikes each surface. One part of the light travels though the surface and
the other part of the light reflects from the surface. If there was not a
reflection then we could not see the lens element, only its effects.
Therefore the total energy of the light striking each surface is divided
into two parts, the transmitted part and the reflected part. Only the
transmitted light creates the picture. If the reflected light can be
reduced then the transmitted light will be increased. The purpose of the
transparent coating or layer is to reduce the reflection of light at the
lens surface thereby increasing the transmission of light through the
lens.
For example if a photographic lens contains five separate
elements then this would cause the light passing through the lens to
strike ten separate surfaces. Zoom lenses often have twelve to thirty
internal surfaces for light to pass through.
Coated lenses were very
rare before the Second World War. Because of the large advances in
technology caused
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