Additional Consideration


blue peacock 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.

Diopter adjustment

binoculars 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 barrel.

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

bird close up 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

fov2 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 of attention".

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

red bird 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 magnification pair.

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.

Weather Proofing

bird over water 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.

Rubber Armoring

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.

Previous Article - Basics II | Next Article - Basics IV

Learn About Optics

Day Optics

Designs - Quality, compacts, porro and roof prism designs for binoculars and scopes...
Designations and Considerations - Designation values, eye relief, weight & cups, exit pupil, and twilight factor...
Additional Consideration - Focusing, field of view, depth of field, weather proofing and nitrogen fill...
Optic Components & Image Quality - Lenses, mirrors, coatings, aberrations, distortions, and alignments...
Spotting Scopes - Construction, Objective lens, eyepieces, angled or straight, and focusing...
Tripods - Heads, legs, monopods, shoulder stocks, and window mounts...
Digiscoping - About, power, editing, considerations, cameras, techniques, and effects...
Care & Tricks - Holding techniques, cleaning, carrying, and protecting your optics...

Night Vision

Starlight Technology - NV Types, Starlight Technology defined, basic design and IR Illuminators...
Starlight Technology Night Vision Generations and Devices - Generation 1 to 4 - levels of NV technology, types of devices and their uses...
Use & Care - How to use, controls, and care for NV devices, extending capabilities...
Digital Night Vision and Thermal-Imaging - Digital NV and Thermal Imaging, how they work and compare to standard NV...

Buying Guide

Binoculars - All the factors to consider when buying binoculars.
Bins for kids - Special Considerations for children's binoculars.
Challenged - Special considerations with binoculars for the physically challenged.
Spotting Scopes - All the factors to consider when buying a spotting scope.
Tripods - Selecting the best tripod for your scope.
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