parts. Birding binoculars have three basic elements: the
front lenses, the rear lenses, and the prisms. The front lenses
are called the objective lenses. They gather the light and
focus an image of the object inside the binoculars. The rear
lenses are called the eyepieces. They enlarge this image and
present it to the eye. The third element are the prisms, located
between the objective lenses and the eyepieces. The prisms
function as mirrors, reversing the image and turning it right
side up. Without the prisms, the birds would appear upside
down and backwards. The prisms also fold the light path so
that the overall length of the binoculars is shortened.
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 eyeglasses.
best? To select the best binoculars, you must choose them
personally. 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.
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.
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 paper. You can enlarge
this image by examining it through another magnifying glass.
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.
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 prisms.
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.
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 were born.
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.
prism vs. roof prism Porro prism binoculars were standard
until the 1960's, when the Zeiss and Leitz companies introduced
roof prism binoculars, whose objective lenses were straight
in line with the eyepieces. Roof prism binoculars were compact,
light, and comfortable to hold. They made the offset, zig-zag
shape of the Porro prism design look as old fashioned as propeller-driven
aircraft. Roof prism binoculars appeared simpler than Porro
prism binoculars. But inside, they had a more complex light
path and required much greater optical precision in manufacturing.
As a result, they cost more to make. The Porro prism design
was simpler and more light efficient, and its images showed
better contrast. Nevertheless, the roof prism design's appeal
was so great that manufacturers went all out to perfect it.
They succeeded. Today, roof prisms dominate the top-end birding
binocular market. Porro prism binoculars are not obsolete,
however. Dollar for dollar, a Porro prism design will give
better performance for the money, especially in medium or
low priced binoculars.
Porro prisms binoculars are made from a high density glass,
BAK-4. If you hold binoculars away from your eyes and up to
the light, you can see the circular exit pupils in the eyepieces.
The less expensive BK-7 prisms will have squared-off, non-circular
Coatings: through a glass darkly Each time light enters
or leaves a piece of glass, about 5% is reflected back. Binoculars
may have 16 air-to-glass surfaces, with light lost at every
surface. In early binoculars, less than half the light got
through to the eye. The rest bounced around inside the binoculars,
making the image hazy and hard to see, like a movie in a theater
with the lights on.
Bigger objective lenses can compensate for the lost light,
but they result in heavier binoculars. In the 1940's, it was
discovered that coating the glass with magnesium fluoride
would let more light through. The original coating technology
was a single layer, which reduced reflections to 1-1/2% per
surface, instead of 5%. More recently, advanced multi-layered
coatings have reduced reflections to as little as 0.25% per
surface. Today, in the best binoculars, 95% of the light gets
transmitted to the eye.
technology depends on applying perfectly uniform thicknesses,
a few millionths of an inch thin. A little too thick or thin,
and the coatings won't work. As you might imagine, the better
the coatings, the more expensive the binoculars.
expensive roof prism binoculars, a special feature to look
for is anti-phase shifting coatings. Roof prism binoculars
with these special coatings on the roof surface will deliver
are some symbols that are used to describe binocular coatings:
(C), coated optics: one or more surfaces coated.
fully coated: all air-to-glass surfaces coated. But if any
plastic lenses are used, they may not be coated.
(MC), multi-coated: one or more surfaces are multi-layer coated.
fully multi-coated: all air-to-glass surfaces are multi-layer
and light Every pair of binoculars is engraved with a
formula, such as "7 x 35" or "10 x 42." The first number in
the formula is the power, or how many times the image is enlarged.
With hand held binoculars, as with most things in life, there
is a practical limit to power beyond which it is not useful.
Depending on the individual, as the power increases, hand
tremor begins to degrade the image. Binoculars over 10 power
usually require tripod mounting.
second number in the formula is the diameter of the objective
lens in millimeters. The bigger the objective, the more light
can enter, and the greater the potential resolution of the
performance is largely dependent on the exit pupil. Exit pupils
are the small, bright circles you see in the eyepieces when
you hold binoculars away from your eyes and up to the light.
They are the actual beams of light coming out through the
eyepieces. The exit pupil is calculated by dividing the diameter
of the objective lens by the power. A 7 x 35 binocular has
an exit pupil of 5 millimeters (35 Ö 7 = 5).
noon, the pupils of your eyes contract to 2 to 4 mm, and at
night they may open to 7 mm. If the beam of light exiting
the binoculars is wider than the pupil of the eye, the excess
doesn't get in: the eye can't see it. During daylight hours
things look just as bright through binoculars with 4 mm exit
pupils as through those with 7 mm exit pupils. In fact, if
they have better coatings, binoculars with 4 mm exit pupils
will be brighter.
in low light that the larger exit pupil is an advantage. For
astronomy, an exit pupil of 7 mm is standard. For birding
purposes a 6 mm exit pupil is usually large enough for even
the most demanding low-light condition. For daylight viewing,
even smaller exit pupils may be more than enough.
we age, the eye loses its ability to dark adapt. While a 20-year-old
person's pupils might open to 7 mm, at 50 years the pupils
may open only to 5 mm. Therefore, binoculars with large exit
pupils may not help the older birder.
best birding binoculars are bright as a result of their advanced
multi-coatings and top quality optics, which provide brightness
you can see all the time, even in daylight.
binoculars with a larger exit pupil improve your ability to
identify birds at dusk? The best way to tell is to try them,
using your eyes.
of view The field of view is how wide an area is encompassed
in the binoculars' image. It can be expressed as an angle
(8¡), or as the width in feet of the image at 1000 yards (420
ft.) To convert the angular field to the linear field, multiply
the angular field by 52.5. Field of view is a matter of eyepiece
design. More power usually means a narrower field of view.
Wide-field-of-view eyepieces usually have shorter eye relief
and may not work for eyeglass wearers. Though it is important,
field of view is usually not the first concern in choosing
Eye relief and eyeglasses Eye relief is how far back from
the eyepiece your eye can be and still see the whole field
of view. It is the most important consideration for eyeglass
wearers, because glasses hold the eyes back from the eyepieces.
If the distance to your eyes is greater than the binoculars'
eye relief, you will see only the center part of the image.
It's like paying for a box seat but watching the game through
a hole in the fence.
Normal eye relief for binoculars ranges from 9 to 13 mm. Even
though the eyecups of most binoculars fold down to let glasses
wearers get closer, in many cases, it's not close enough.
If you wear glasses, you need binoculars whose eyepieces are
specially designed with long eye relief, which manufacturers
describe as 14 mm or longer.
It's odd that eye relief specifications are often left out
of brochures and catalogs, since the eye relief number is
the best way to compare models for use with glasses. To measure
eye relief yourself, point the binoculars' objectives at a
bright light source and move a paper back and forth near the
eyepieces. The eye relief is the distance from the eyepiece
at which the circle of light comes into sharpest focus. This
is the optimal place for the eye to be. It's called the eyepoint.
If your binoculars' eyepoint is not compatible with your eyeglasses,
you are missing the big picture, a lot of the fun, and possibly
eyeglasses You may find it helpful to buy a special pair
of birding eyeglasses that let your eyes get closer to the
eyepiece. For example, my "spare" eyeglasses sport old-fashioned
wire-rim frames. The lenses are much smaller and lighter than
my regular frames. They bring my eyes closer to the eyepieces
than my regular glasses and give me more choice of which binoculars
I can use. I can even use 10 x 25 pocket binoculars witIhout
feeling as if I'm trying to look through a pair of soda straws.
f your regular prescription includes bifocals, ask your optician
to move the dividing line as low as possible, so that you
can look over it when using your binoculars. Don't be tempted
to eliminate the lower bifocal entirely. You will regret it
the first time you try to read your field guide.
The anti-reflective coating technology used on binocular optics
is also available for eyeglasses. It increases light transmission
when using your glasses with binoculars. It will also improve
visual comfort, especially at night, while working under fluorescent
lighting, or when using a computer. Once you try it, you'll
want it on your regular frames too.
to choose binoculars Buy the best. Superior optics really
pay off for birders in the quality of experience they provide.
They will stand up to heavy use and keep their resale value.
It's a false economy to buy less.
before you buy. Binoculars that are perfect for somebody
else may not be the right binoculars for you. See if you can
borrow a model you are interested in from a friend and bird
with it for a day. Or ask the dealer if you can take two or
more models into the field for comparison. If you haven't
pre-tested a particular binocular, don't buy it without the
assurance that you can trade it in for a different one.
human eye has a great ability to compensate temporarily for
slight misalignment or focus problems. For the few minutes
spent evaluating models in a store, a pair of binoculars may
look fine. But after an hour in the field you may begin to
experience eye fatigue or even a headache. Subtle differences
between binoculars may take some time to show up.
best binoculars will disappear from your awareness while you're
using them, so that your attention is on the bird, not the
binoculars. The mark of good binoculars is that they make
you feel as if you are simply seeing through your own eyes,
only closer. You can look through good optics all day long
with no sense of strain. With inferior optics, you feel a
subtle sense of relief when you stop looking through them.
pays to invest in the instrument you really want. You will
never be sorry you bought the best binoculars.
to compare models Indoors, where you can control lighting
conditions, tape a dollar bill to a wall. Then compare how
well various binoculars resolve the same details, both at
the center and at the edges of the visual field. Test the
binoculars without support, hand holding them as you would
in normal use. Is one pair more comfortable to hold? Do you
prefer looking through it? You can also test binoculars on
a tripod or a beanbag support. You may obtain different results
from what you find in the hand-held test.
them in the field. Consider the weight. After an hour
are they still comfortable around your neck? You may want
to try using a replacement strap made of neoprene rubber.
Wide and stretchy, it helps to absorb shock and protect the
to avoid in birding binoculars Zoom binoculars are usually
optically inferior to regular binoculars. Avoid fixed focus
binoculars. They won't focus up close. And don't get binoculars
that focus each eyepiece separately. Individual focus is appropriate
for marine binoculars, which require complete waterproofing,
but it's too slow on the draw for birding.
are the best birding binoculars? It's really a matter
of personal choice. Different models are best for different
purposes and different birders. The 10 power that's tops for
distant shorebirds is outperformed by a 7 power with a wider
field of view when searching for warblers in dim light.
personal pick If I had to pick the best one-size-fits-all
birding binoculars, they would be the 7 x 42 Zeiss B/GA T.
With 7 power magnification, they are universal binoculars
that everyone can hand hold. The slim, tapered shape fits
large or small hands. Resolution and color accuracy are unsurpassed.
The optics are incredibly bright, and a 6 mm exit pupil gives
excellent low-light performance. They focus close - 11 ft.
An 8.6¡ field of view delivers a big picture, and they're
wonderful, wonderful with eyeglasses. This rugged instrument
is armored and won't fog up in wet weather. At 28 oz. it may
be a tad heavy for some, but the performance is worth the
weight. Their only flaw is that the diopter adjustment is
easily turned off its setting. However, other users might
consider the ease of re-setting the diopter adjustment a desirable
priced birding binoculars Included in the table are two
examples of very low priced binoculars, the 7 x 35 Bushnell
Birder and the 9 x 25 Nikon Travelite III. They get high ratings
for the quality of their optics considering their cost. For
example, the Bushnell Birder can be bought from some mail-order
houses for under $40, including a case and a beginning birdwatching
book. No one need be without a loaner.
20 x 60 S stabilization binoculars Birding machismo occasionally
requires some birders to boast that they can hand hold 20
power binoculars or a spotting scope. The new Zeiss 20 x 60
S stabilization binoculars make this dream come true for everybody.
Push a button, and the prism system inside is free to sway
with the movements of the hand. The image stops shaking. How
stable is it? Zeiss claims that you can resolve a 15 mm object
at 1000 meters without using a tripod. A dime is 18 mm.
magic is all done mechanically, without any electronics. No
whirring gyros to spoil the silence, and no batteries to run
down. Good news for eyeglass wearers: the 20 x 60 S offers
14 mm of eye relief, outstanding for 20 power binoculars.
The binoculars are rubber armored and come in a high-tech
aluminum case. They weigh 3.5 lbs., but this is feather light
for typical 20 power binoculars. Especially considering you
can leave the tripod behind. And think of the speed! How many
times have you missed a bird while setting up your tripod
and scope? I'll bet that, even at $4725 a copy, Zeiss can
sell all they can make.