Ease and sharpness of focusing
are important factors to consider when choosing a binocular or scope.
There are several factors to consider when evaluating focusing. Most optics
use a focus knob that you turn with one finger while looking through the
optic. As a general rule, the knob should make one full revolution of
travel from one extent to the other.
Getting the sharpest focus
possible may require very fine adjustment. The best optics deal with this
in several ways. One method is to increase the number of revolutions that
it takes to focus, in effect making the adjustment finer. Though birders
say they get used to this, it slows down the focusing process and confuses
users about which direction to turn the knob since it takes more travel
to see if the image is becoming more or less focused. Another problem
with binoculars requiring more than one revolution of the focal knob to
travel between focal extremes is the amount of time required to focus.
Extra time needed to focus can mean the difference between getting a good
view of a moving bird or not and you may miss some birds altogether.
Other manufacturers use a “variable-speed”
focus dial, where rapid turning of the knob yields coarse focal adjustment
while slow turning switches it to fine focal adjustment. This system works
very well, but takes time to get used to in the field. A third solution,
more commonly found on scopes than binoculars, is the use of two separate
focusing knobs, one for coarse and one for fine focusing.
Another consideration with
focusing is the ease and smoothness of the system. The focal knob should
comfortably be positioned under your index finger as you hold the binoculars,
or easily located on a scope without removing your eye from the eyepiece.
It should operate evenly and smoothly without any looseness, stiffness
or irregularities of movement.
All binoculars should have
a diopter adjustment to allow the two barrels to be set at sharp focus
for both eyes simultaneously. It is a critical part of setting your binoculars
to match your individual eye strengths. The adjustment is only on one
of the two barrels so that you can bring it to equal adjustment with the
other. Since one of your two eyes is often a little stronger or focuses
a little different than the other it is very important to make this adjustment.
The adjustment should be done the first time you use a binocular and then
checked every so often.
The diopter adjustment ring
is usually located on the right eyepiece so it can be adjusted. Some of
the newer models have incorporated it into the center focusing wheel and
you must pull out the focusing wheel to make the diopter adjustments.
The diopter adjustment method will be explained in the documentation.
The adjustment should not be sloppy and should maintain its setting so
you don't have to continually readjust it.
The method of making this adjustment
is quite simple and quick. First choose an object about 50 to 75 feet
away to focus on. Close the right eye or block off the right barrel (put
on the lens cap) and using the focusing wheel bring the object into sharp
focus. Once the object is in sharp focus, you might take your binocular
away from your eyes, with both eyes open, for a moment to relieve any
eye strain. Now with your left eye closed, use the diopter adjustment
only (do not adjust the focusing wheel) to bring the right eye into sharp
focus. This method is reversed if the diopter adjustment is on the left
To restate this: only one side
(or barrel) of the binocular has an diopter adjustment. Focus the non-adjustable
side to sharp focus with the main focus wheel, using only that eye and
on a specific object 50 to 75 feet away. Now with only the eye open on
the adjustable side, not moving the main focus wheel and looking at the
same object, bring the object into sharp focus using only the diopter
adjustment. Now both sides of the binoculars are adjusted to come to sharp
focus for both of your eyes at the same time.
Close focus refers to how close
you can bring an image into sharp focus. It is usually specified or can
be easily checked. It is an important part of choosing a binocular or
a scope, particularly if it is ever used to take photos (see digiscoping).
It may seem that anything closer than about 15 feet doesn’t require
a binocular view... this is not true. Birds may come in much closer than
this, or you may simply wish to examine details of plumage in greater
detail. You may also want to examine other things like butterflies, reptiles,
flowers, etc. and close-focusing optics are advantageous in such situations.
A 6 to 8-ft close focus is
an advantage in a binocular not only to focus on small close objects,
but also because it gives you the ability to move beyond sharp focus in
both directions while focusing. This helps to get a very sharp view. Traditionally,
as magnification increases, the minimum close focal distance also increases.
This generally holds true though there are now mid and high-end binoculars
that close focus to 5-8 feet even at 10x or 10.5x magnification. Still,
if you choose a high magnification binocular you may have to compromise
your ability to close focus. Even some high-end optics may not close focus
any closer than 15-16 feet, which is something of a limitation in our
view... Any “close focus” longer than this is a handicap.
Field of View
When looking around with your
eyes, your view comprises the full area from the ground to the sky and
about 160-170° horizontally of that which is in front of you. In practice,
you only actively concentrate on a small portion of this whole view. We
will define this small portion of active concentration as the "area
When looking through optics,
your view is reduced to a narrow cone of light defined as the field of
view (FOV). This cone ranges from about 5 - 8.5 degrees. The triangle
in Figure #1 shows the FOV angular measurement. In essence, we lose everything
beyond our "area of attention". Consequently, when looking through
optics, you must physically move to see, where looking with just your
eyes you can simply refocus at a different point in your view before moving
your eyes or head.
FOV is often defined by what
the total horizontal view, in feet, is at 1000 yards distance (see Figure
#1). In general, the field of view decreases as magnification increases.
This is because you are, in essence, "zooming in" when you increase
magnification. The effects of this are two fold. First, as FOV decreases,
it is more difficult to track moving objects. Following a moving warbler
is more difficult with 10x binoculars than with 8x. Second it can be more
difficult to find a bird especially at a distance since your initial "aim"
needs to be better. However, a bird in a bush may also stand out better
at higher magnification and not disappear into the leaves as much.
The FOV varies to a limited
degree between different binoculars even of the same magnification. This
is largely determined by the eyepiece. In general, a wider than normal
FOV has some trade offs, most notably a shortened eye relief distance
(your eyes need to be closer to the eye piece causing eye glass wearers
problems) and increased cost. Secondary consequences include higher weight
and greater optical distortion (see "Basics
IV - Optics").
Depth of Field
When looking around with just
your eyes, everything is in focus from about 10 feet out to infinity.
Your eyes also automatically refocus as you look at closer objects. Looking
through optics is much different in that the focusing knob must be adjusted
as objects move closer and farther away to keep the image in sharp focus.
The depth of field is defined
as how much depth of your view is still in sharp focus in front of and
behind that which you are looking at. This is most prevalent at close
distances i.e. the depth of field is much greater at 20 yards than it
is at 20 feet. In general the depth of field also decreases as magnification
increases. In other words, the depth (distance away from you) that objects
are in sharp focus is greater with 8 power binoculars than with 10 power.
The depth of field is defined
as how much of your view is still in sharp focus in front of and behind
that which you are looking at. Depth of field increases proportionally
with distance: your depth of field is much greater at 20 yards than it
is at 20 feet. In general the depth of field also decreases as magnification
increases. Thus, the depth (distance away from you) that objects are in
sharp focus is greater with 8x binoculars than with 10x. In practice,
when a bird is close to you in a bush and he moves about, his distance
from you changes. You will have to adjust your focus more often with a
10x binocular than with an 8x. It may also mean that if you have 2 birds
in your view at different distances, both may not be at sharp focus simultaneously
with a binocular of higher magnification where they may be with a lower
To some people, the need to
"sharpen" or adjust the focus on their binoculars is a source
of fatigue and they will want to choose optics of lower magnification
and greater depth of field. I am personally comfortable with adjusting
focus and it becomes a natural part of looking through optics to gently
adjust the focus to bring images in sharp.
The difference in depth of
field between 8 and 10x optics is actually not all that great to the average
birder and may not even be noticed. In a direct comparison the differences
in magnification are going to be much more obvious. This is also mostly
applicable only at close ranges since at a greater distance the depth
of field becomes much larger.
If you are doing anything beyond
looking at the birds out your kitchen window, weather proofing isn’t
an option – it’s a necessity. We generally mistrust weather proofing
of binoculars with external focusing (see also "Basics
I - Designs"). Although the manufacturer may claim these are
weather proof or resistant, they are sealed with an "o" ring
and over time these can wear, dry out or get damaged. The result is that
moisture and/or dust can get into the interior, settle on the optics and
degrade your view. External focusing binoculars are normally quite obvious
since the eyepiece moves in and out as you adjust the focus. The internal
pressure also changes while focusing and using this type of optic. This
further increases the possibilities of dust and moisture being pulled
into the system.
While using binoculars in the
field there are just too many unforeseen possibilities for unwanted "stuff"
getting inside your optics if they are not weather-proof or resistant.
The difference between these two designations is as follows: a weather-resistant
optic is one that will withstand the occasional light shower or fog but
will not withstand submersion or extended use in wet conditions. The manufacturer
may not repair these if they fog internally. A claim of weatherproof (or
water-proof) optics means that the manufacturer will guarantee the optics
against water intrusion under most circumstances. Sometimes they are even
rated to a depth of submersion although we don't discuss watching fish
underwater in this article.
Weatherproof binoculars are
a wise choice for most active birders. This generally protects your binoculars
from the unforeseen slip into a stream you're crossing over, unexpected
rain or even getting them wet while using them in the snow. It generally
implies greater durability although no optic will withstand continuous
abuse. The option must be found in the specifications of the optics and
is not visible... such as with the presence of rubber armoring.
Nitrogen or Dry Gas Fill
The purpose of filling an optic
with (usually) nitrogen is to prevent interior moisture condensation.
Neither high price nor a claim of being sealed guarantees that your optics
are nitrogen-filled. The problem results from going from a warm to a cold
environment or from subjecting a cold optic to warmer air (such as your
own breath). Water condenses in the interior and the optics fog up. Not
only does the fog make it really difficult to see through your binocular,
but you can eventually get mold growing internally if the water isn’t
removed promptly. Nitrogen-filling is generally standard with sealed optics
but not always. There are some $700 - $800 optics on the market that are
not nitrogen-filled. It is something to check. If you bird in cold weather,
it is a nasty surprise to get out and find your optics have fogged up.
Modern binoculars and scopes
are often clad in rubber armoring. Rubber armoring provides protection
against corrosion and dirt, and helps cushion blows to the instrument.
There are several types of coatings and designs for the outside shell
and contour on binoculars. Modern rubber armoring is generally made from
urethane that is reasonably resistant to bug repellants (unlike most eyecups
that are often made from natural rubbers). It also has a comfortable feel
over time and prevents scratches to metal surfaces. The only disadvantage
of rubber armoring is that it adds to the weight of the optic.
- Focusing, focusing speed
and fine focus are a critical part of optics. There are multiple options
and considerations in dealing with these concerns
- Close focus is a useful option.
A close focus of under 10 feet is very nice, 10-16 feet is acceptable,
but beyond 16 feet limits versatility and functionality.
- "Field of View"
is generally defined as the horizontal width, in feet, of the view as
seen at 1000 yards.
- "Depth of Field"
is the range of distance behind and before the object being viewed that
remains in sharp focus.
- Weather or waterproof optics
have better manufacturer warrantees for repair and will withstand most
normal field conditions.
- Nitrogen- (or other dry gas)
filled optics prevent internal moisture condensation (fogging) and are
essential for cold weather birding.
- Rubber armoring is a nice
option that helps to protect your optics and provides a comfortable
feel, at the cost of some additional weight.
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