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made about binoculars and
brightness and light loss.
Guides to purchasing optics
expounded upon the mysteries
of the “twilight factor," and
“relative brightness," and
“relative light efficiency." Sales
people who know little about
binoculars except, perhaps, how
to read charts still expound
upon these mysteries.
Certainly a bright image,
offering sharp contrast and
accurate color rendition is
important to birders – in fact,
it is essential. The fact of the
matter is that since the advent
of coated lenses…and multi-
coated lenses...and phase
coated Roof prisms...all the old
rules governing glass and light
have been bent.
If you are willing to pay the
price for quality binoculars,
you are virtually assured of
owning binoculars that offer
superior brightness.
The problem used to be this.
Every time light strikes polished
glass (like a lens; like a prism)
5% of it is lost, reflected away.
In every binocular there are 10-
16 glass surfaces whose
cumulative loss of light equaled
about 50%. The net result was
a dark image.
In partial compensation,
binocular manufacturers could
increase the size of the
objective lens. The size of the
objective lens is measured in
millimeters and appears as the
second number of the legend
stamped on binoculars
(example: 7x42, 8x30, 10x50).
Larger objective lenses allow
more light to enter the
binocular, increasing the
diameter of the shaft of light
that exits the binocular to fall
upon the human eye.
This shaft of light, seen as
the bright dot swimming in the
center of the ocular lens is
called the “exit pupil" and is
still an important
consideration when buying
optics. Here’s why.
The human eye has a pupil,
too – one that opens and
contracts in response to light
conditions. In bright light it can
expand to about 7 mm
(depending on your age), thus
establishing the functional limit
of the binocular’s exit pupil –
because any light falling
outside the maximum limit of
the human eye serves for
nothing.
Beginning in World War II, it
was discovered that a coating
of reflection reducing material
(like magnesium fluoride)
applied to the surface of glass
could reduce light loss caused
by reflection from 5% to almost
1%. This coating appears as a
blue, or purple, or green glaze
on the lenses.
Later it was discovered that
by applying multiple, thin
coatings instead of a single
coating, light loss could be
further reduced to a mere
fraction of 1%.
Recently, several
manufacturers of superior
quality Roof prism binoculars
have begun coating the “Roof
prism" with a material that
compensates for the modest
wave length shift inherent in
the Roof prism design with the
result that “phase-corrected"
Roof prisms offer the same
sharp image contrast found in
Porro prism binoculars.
The important consideration
when buying binoculars is to
make sure the optics are “fully
coated" or, better still “fully
multi-coated." The operative
word is “fully" because this
means that all air to glass
surfaces both inside the
binocular and out have been
treated to reduce light loss.
Different manufacturers use
different, patented names to
distinguish their coatings. The
important thing is to be sure
that all glass surfaces, inside
and out, are single or “multi-
coated" to reduce light loss.
The process used to coat
binoculars lenses is exacting
and costly. It accounts for
much of the price difference
between quality binoculars and
less expensive binoculars. But a
commensurate price can also be
your assurance that your are
buying binoculars that are
bright and sharp enough to
perform in the field.
Field of View
and Depth of Field
Field of view is the measure of
the distance from one side of a
binocular’s image to the other
as seen through a stationary
binocular.
This measurement, either
inscribed on the binocular or
included in the accompanying
literature, may be designated in
degrees of arc (ex: 6, 7); feet at
1,000 yards (ex: 415 feet at
1,000 yards); or meters at
1,000 meters.
However the measurement is
noted, a wide field – one that
offers no less than 6 degrees
of arc (or a minimum of 300
feet at 1,000 yards) is
essential to birding.
