And How They Work
by Michael and Diane Porter
What birders need
Birders demand a lot from their binoculars. Birding binoculars must be
light enough to carry all day long and sturdy enough to survive years
of heavy use. They must be easy to hold steady. They must resolve delicate
details and reveal subtle colors with accuracy. They must focus quickly
and up close and work well in dim light. They must be sealed from dust
and moisture. And they must show the whole picture even for birders wearing
When choosing binoculars, it's best to try them out them in person. Only
you can decide how much magnification you can hand hold steadily and how
much weight is comfortable to carry. Only you can feel how binoculars
fit in your hands and how well they work with your eyeglasses.
To make the best
choice you also need a basic understanding of how binoculars work, so
that you won't be confused by the technical data. Knowledge is the compass
that lets you navigate the informational sea.
How binoculars work
Essentially, binoculars are just two telescopes mounted side by side,
one for each eye. To understand binoculars, you need to understand how
a telescope works. Here's an easy demonstration that you can try yourself.
All you need are two ordinary magnifying glasses and a piece of tracing
paper. Do this once, and you will understand forever how binoculars work.
Hold the tracing
paper on the opposite side of the magnifying glass from a bright object,
such as a light bulb. Move the paper back and forth. At a certain distance,
an upside-down-and-backwards image of the light bulb will form on the
You can enlarge this
image by examining it through another magnifying glass, as shown in the
illustration. You may be surprised to find that if you slide the tracing
paper away, the image will remain, only brighter and clearer. You have
just made a working telescope.
The magnifying glass
nearest the object is called the objective lens; the one nearest your
eye, the eyepiece. The objective lens and the eyepiece are two elements
in all binoculars. Binoculars also have a third element, the erecting
In the telescope
we just built, everything is upside down and backwards. That would be
OK for looking at stars, but for watching birds or following the action
at a football game we require a right-side-up picture. A terrestrial telescope
has to flip the image, and that's what prisms do.
A prism is a solid
piece of glass that functions as a mirror, but without a mirror's reflective
backing. Light rays that have entered a prism cannot get out if they strike
a surface at too great an angle. Instead, they reflect back, as if from
a perfect mirror.
In the mid 19th century,
an Italian named Porro designed a telescope with two prisms set at right
angles to each other between the objective lens and the eyepiece. This
arrangement not only erected and reversed the image, but also folded the
light path, resulting in a shorter, more manageable instrument. In 1894,
the Zeiss Optical Works created the first "Hunting Glasses,"
incorporating the Porro prism design, and modern prismatic binoculars
All binoculars still
have these three parts. An objective lens focuses an upside-down image.
A set of prisms turns the image right side up. And an eyepiece magnifies
it. Though modern eyepieces and objective lenses are each comprised of
multiple elements, their basic functions remain unchanged.
Today you can buy
binoculars that are made with roof prisms or Porro prisms. There are advantages
—Michael and Diane Porter
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