 
Lens elements - Optical building blocks
A photographic lens can be just a single piece of glass curved in such a
way that at a certain distance (called its focal length) it forms an
image. Lenses for very simple camera are no more than that. However more
advanced cameras require more sophisticated optics, lenses combining
several individual pieces of glass in one unit.
Why use many elements instead of just one in a high quality photographic
lens? Certainly a one element lens is cheaper to make, easier to mount
in a barrel, and lighter to carry around. Any element used alone,
however, also exhibits many optical defects called aberration. They
prevent a single element from forming a sharp, crisp image. Therefore,
to improve their performance as much as possible, one element lenses are
designed so that only a small central portion is allowed to pass light
rays. This helps image quality to a degree but makes the lenses so slow
that action or dim light photography becomes difficult.
When additional elements are combined in one lens, aberrations can be
tamed and maximum aperture widened. The more elements, the more control
over defects. However, there is an increasingly severe penalty to pay as
well. As elements are added, incoming light causes more reflections
between the various airs to glass surfaces. The result is flare and
decreased image contrast. Equally unfortunate is the fact that the loss
of light transmission through several elements can add d up to as much
as 70 percent.
Before the invention of antireflection lens coating designers tried to
overcome these difficulties by eliminating as many air/glass surfaces
within a lens as possible. They did this by keeping the number of
elements small and cementing them together ring twos and threes. Today's
lens designers are largely free from these restrictions because of
lens-coating techniques. When elements are covered with one or more
ultrathin layers of calcium or magnesium fluoride, much of the
reflection is eliminated and light transmission is greatly improved.
Therefore, in modern lenses containing many elements most components
stand alone, separated from the others by carefully computed air spaces.
These air spaces act somewhat like elements in themselves and give
optical engineers much more flexibility in lens design.
Most lens elements are made of high quality optical glass. A complex
lens unit may contain components produced from several different types
of material, such as flint glass, crown glass, barium glass, or the so
called "rare earth" glasses. For utmost quality in certain telephoto and
other special purpose lenses, fluorite crystal and quartz elements may
be used. The reason for employing such a wide range of materials is that
each one has a different index of refraction; that is each one bends
light rays differently. Some moderately change the direction of a beam
of light, while others bend it at a much greater angle.
Thus, by using several elements made of various types of glass, each one
curved and shaped in its own special way and each one separated from the
others by a precisely calculated distance, designers can create complex
lenses in which aberrations are largely eliminated. Lenses can be made
both sharper and faster. Furthermore, the ability to apply modern
antireflection coating before combining various types of elements like
optical building blocks has made possible advanced designs such as super
wide angle lenses, mirror telephotos and zooms.
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