How To Choose Binoculars

How binoculars work: Galilean Optics.
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Binoculars are probably the world's most used optical instrument. That's not to say that telescopes don't have their place, but binoculars are king. Binoculars are optimized for a lot of different uses, and choosing the right pair will depend heavily upon your intended use. The purpose of this guide is to help you understand these instruments a little better so that you can make an informed decision when choosing a pair for yourself. Refer to our comparative Binocular Review to see which models out of 11 top sellers we liked best and why.

What Are Binoculars?

Binoculars are basically two telescopes hinged at the center, allowing the eyepieces to fit the width of your eyes. Before we can understand binoculars we need to understand how a telescope works. A telescope is made up of two major pieces: the objective lens and the eyepiece. The objective lens is the lens nearest the object. Its job is to gather and focus light. The eyepiece is the lens next to eye. Its job is to magnify the image. The amount of magnification is determined by the focal length. The focal length is the distance between the eyepiece and the focal point, or the image created by the objective lense.

A good way to understand these concepts is to take a magnifying glass and a piece of paper. Point the magnifying glass towards a brightly lit object and slowly move the piece of paper towards the magnifying glass until the object comes into focus. That point is the focal point where the light rays converge, the magnifying glass being the objective lens. If you used a light piece of tracing paper and then took another magnifying glass, the second magnifying glass would be the eyepiece. You could remove the tracing paper and the image would still appear when you looked through the eyepiece. You have now made a telescope! The only issue with this is that the image is inverted and backwards, which is one of the downfalls of your basic telescope.


What separates binoculars from a basic telescope is the use of a prism to flip and rotate the image. Let's go back to our double magnifying glass example, but this time use two porro prisms at right angles in between the lenses. The first prism will flip the image and the second prism will rotate the image. Now our object appears normal when looking through the eyepiece. One of the advantages of the prism is that it folds the light, creating a longer objective focal length. This allows for greater magnification in a smaller length.

Now that we know the role the prisms play, there are two types of prims commonly used in binoculars that we need to understand. These two types are: porro prism and roof prism. The porro prism is the example used in our magnifying glass experiment. The roof prism in binoculars allows the light stream to enter and exit on the same plane.

How binoculars work: prism optics. Roof prism vs. Porro-prism construction.
How binoculars work: prism optics. Roof prism vs. Porro-prism construction.

The porro prism style can further be broken down into two types: the traditional style which has the objective lens further apart than the eyepiece, or a reverse porro prism where the eyepiece is further apart than objective lens.

The differences between the different types of prisms are going to be more noticeable in the lower to mid-range binoculars. Porro prism models are bulkier than the roof prism models, but also require less strict tolerance to produce. This means they are easy to manufacture and cost less to make. Some people find the porro prism binoculars more comfortable to hold. The roof prism allows binoculars to be more compact and narrower than porro prism models. The trade-off is that a roof prism needs a phase correction coating, is more prone to alignment issues, and if a cheap silver coating is used, you could lose up to 13% of the light coming into the system. You are going to pay more for a pair of roof prism binoculars with comparable optical quality to a pair of porro prism binoculars.

Prism Materials

Two types of materials are used in manufacturing prisms: BK-7 (borosilicate) glass and BaK-4 (barium crown) glass. Prisms made of BaK-4 are preferred over BK-7 because they have a higher refractive index and give brighter and well defined images. It is said that you can tell the difference between BK-7 and BaK-4 by holding the binocular's lenses towards a light source and looking at the exit pupils. If the prisms are made of BaK-4 glass, the exit pupils will be round and evenly illuminated. If the prisms are of BK-7 glass, you will notice squarish, gray edges in the exit pupils.

Glossary of Terms

Knowing your terminology is important, so let's go over a few thing you should know. The first thing you're going to see is some numbers like this: 8X42. The first number is the magnification, the second number is the objective lens size. We will talk about these two items first.
  • Magnification is how large the image will be through the binoculars vs. your naked eye. In the example of 8x42, an image would be 8 times larger than what you would normally see. Magnification also impacts the depth of field, your field of view, and the overall brightness of the image. Magnification also magnifies movement. The higher the power, the harder it is to hold steady. Above 10x you will probably need a tripod system. We discuss depth of field and field of view in more detail below.
  • Objective Lens Lens size determines how much light enters into the binocular and is transmitted to your eye. In our example above, the objective lens is 42mm in diameter. A larger objective lens will allow for better viewing in low light with a similar magnification. A larger objective lens has a better potential for increased detail and color resolution.The increased detail is due to the size of the airy disc and the airy pattern. An airy disk is the center bright circular region created when light is diffracted through a small circular aperture. The central disk is surrounded by less intense rings or airy pattern. Since your binoculars are one big aperture focusing many dots of light, the more overlap of the airy pattern, the less detail. Though the biggest determining factor is the optical quality. A larger objective lens provides a sharper image at a distance with similar optical quality. A large objective lens uses more glass and therefore will be heavier. 50mm is considered the limit for carrying around.

The objective lens and the eyepiece on the Celestron Cavalry.
The objective lens and the eyepiece on the Celestron Cavalry.
  • Exit Pupil: magnification and the size of objective lens determines the size of the exit pupil. You can see the exit pupil by holding the binoculars at arm's length and looking at the eyepiece. That round dot of light is the exit pupil. To compute the exit pupil, you divide the diameter of the objective lens in millimeters by the magnification. For our 8X42 binoculars we would divide the objective lens 42mm by the magnification 8, or 42/8 for an exit pupil of 5.25. If we look at a pair of compact binoculars, say a pair of 8X25, we would have an exit pupil of 8/25=3.125. Why does this matter? It doesn't as long as there is enough light so that the pupils of your eyes are smaller than the exit pupils of your binoculars. Some articles claim that this is wasted light because your iris is blocking some of the light. We like to think of it as unused potential light. With the 8x25 binoculars in the example above, when it gets dim enough for the pupils of your eyes to exceed 3.125mm in diameter, the binoculars are restricting the light available to your eyes. A healthy young adult's eyes can achieve about a 7mm pupil opening, so a 3.125mm exit pupil from your binoculars can be quite limiting in dim light. In broad daylight the average adult has a dilated pupil of 3mm. The diameter of a healthy young adult can range be between 2mm to 8mm, and as you get older it can get reduced to 5-6mm. A smaller exit pupil also makes it harder to center your eye on the image. Your eye needs to be centered exactly over the exit pupil to see. If your eye's pupil is dilated to 3mm and your binoculars' exit pupil is 3.125mm, this could make it hard to use on a pitching boat or to see a fast moving animal. A larger exit pupil can also help provide eye relief and help people with eyeglasses. Just remember the exit pupil is not a measure of brightness. The quality of the optics can affect brightness more than the exit pupil.

  • Relative Brightness is the exit pupil squared. The relative brightness has no relation to how bright a pair of binoculars will be. The quality of the optics have more to do with the brightness of the binoculars then exit pupil size.
  • Twilight Factor is the theory that small bright images can have as much detail as large dim images. Here is the best explanation of the theory behind twilight factor: let's say you're reading a newspaper. If you're in low light, you have to hold the paper closer (increased magnification). If it's bright out, you can hold the paper further away (decreased magnification). Twilight factor is computed by multiplying the objective lens diameter by the magnification and then taking the square root of this product. Thus a 8x42mm binocular has a product of 336 (8 X42=336). The square root of this is 18.33 (√336=18.33). This number may seem reasonable when comparing a 8x42 pair of binoculars that has a twilight factor of 18.33 with an 8x25 pair that has a twilight factor of 14.14. The twilight factor really only comes to play when looking for binoculars to use during twilight or for astronomy. Again, this formula also doesn't take the optic quality into consideration that has tremendous impact on the perceived brightness of the optics.
  • Eye Relief is the distance the eyepiece of a binocular can be held away from the eye and still present the full field of view. This is important because longer eye relief reduces eye strain and makes it easier for people who wear glasses. The longer eye relief would provide a full field of view for someone who wears eyeglasses. A rule of thumb is, if you wear glasses, you'll need at least 15mm of eye relief.
  • Diopter is the adjustment that allows you to match your individual eye strength. This allows a sharp image in both barrels of the binoculars if you have eyes with differing clarity of vision. To adjust your diopter, use the following instructions: 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. Though most diopters are on the eyepiece, some are integrated into the main focus wheel. If you have any questions on how to adjust your diopter please refer to the manufacturer's documentation.

The diopter knob is located on the eye-pieces and is adjustable separately for each eye.
The diopter knob is located on the eye-pieces and is adjustable separately for each eye.
  • Field of View/Angle of View is the diameter of the circular viewing field. It is defined by the width in feet or meters of the area visible at distance in yards or meters (usually a 1000 yards in the USA). Angle of View is the maximum view you are capable of seeing through the binoculars expressed in degrees. You can easily convert from one measurement to the other by knowing that one angular degree is equivalent to 52.5 feet at 1000 yards. Multiply 52.5 by the angle of view to find the linear field of view. Or divide the linear field of view to get the angle of view. The field of view decreases as magnification increases. As field of view decreases, it is more difficult to track moving objects. Following a moving object with 10x binoculars is more difficult than with 8x. It can be more difficult to find a object at a distance with a 10x pair since your initial aim needs to be better. However, an objects stand out better at higher magnification, especially stars. The eyepiece can also provide a wider field of view. These are special wide angle binoculars. With wide angle binoculars you have a short eye relief along with added weight and optical distortion on cheaper models.
  • Depth of Field is how much of your view is still in sharp focus behind and in front of the object. Depth of field increases proportionally with distance: your depth of field is much greater at 20 yards than it is at 20 feet. Depth of field also decreases as magnification increases. In practice, when an object moves and it's distance from you changes. You will have to adjust your focus more often or if your looking at two objects, one might be out of focus.
  • Optical Quality describes the mounting, the lens coating, and the glass type used. These things will affect the quality of the image and the brightness more than anything else. This is what separates a good pair of binoculars from an inferior pair. When light enters the optics it needs to be focused onto an image with minimal loss to obtain as bright and sharp an image as possible. As the different colors of light (think colors of the rainbow) pass through the lenses, they bend at slightly different angles, similar to a prism or chromatic aberration. Different lenses have different shapes, types of glass, and designs that are used to get each of the basic colors to focus correctly at the right point. Uncorrected light produces blurred images with distortions and muddied colors. Optic manufacturers use very dense (ED, HD, SD) glass and minerals such as fluorite (FL) to help limit the amount the light waves bend. These bending light waves are called aberrations. Other examples of aberrations are astigmatism, spherical and coma.
  • Optical Coatings: When light strikes a glass surface, up to 5% of the light could be reflected and scattered away from your eyes. With up to 20 glass-to-air surfaces, that can be a significant amount of light lost to scattering. Current optical coatings can reduce this to less than a quarter of a percent. Coating the prism adds cost, as does the quality of the coating. The coating needs to be a few millionths of an inch thin and uniform. A little too thick or thin, and the coatings won't work effectively. When comparing coatings you are going to see the following terms:
  • Coated (C) Optics: a single anti-reflective coating on at least one lens surface.
  • Fully Coated (FC) Optics: multiple anti-reflective coatings on at least one lens surface.
  • Multi-coated (MC) Optics: a single anti-reflective coating on all air-to-glass lens surfaces.
  • Fully-multi-coated (FMC) Optics: multiple anti-reflective coatings on all air-to-glass surfaces.
  • Focusing: Ease of focusing is an important factor to consider when choosing a pair of binoculars. When evaluating focusing, most binoculars use a central focusing knob that you turn while looking through the optic. Getting the sharpest focus possible may require very fine adjustment. This can be done several ways. You can increase the number of revolutions that it takes to focus, making the adjustment finer. This slows down the focusing process. Extra time needed to focus can mean the difference between getting a good view of a moving object or not. Variable-speed focus allows rapid turning of the knob to yield coarse adjustment while slow turning yields fine adjustment. This system works very well, but takes time to get used to. The use of two separate focusing knobs, one for coarse and one for fine focusing, can also be used, though uncommon. 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. It should operate evenly and smoothly without any looseness, stiffness, or irregularities of movement. Focusing can also be internal or external. Internal systems use a lens element inside the system that moves back and forth. External systems move the eyepiece back and forth. Independent focusing systems have a focal ring at each eyepiece and allow you to manually focus each eye. The independent focus system is usually found on large binoculars used for astronomy or military purposes. There are "focus-free" or "fixed-focus" binoculars that have no focusing mechanism other than eyepiece adjustments that are meant to be set for the user's eyes and left fixed. Fixed-focus binoculars are great for action because you don't need to adjust your focus. The focus is simply pre-set at about 40 ft to infinity. Fixed-focus binoculars require a large depth of field, which requires a lower magnification, such as 7x.

The focus knob is located in between the barrels  and will assist in improving the clarity of the image.
The focus knob is located in between the barrels, and will assist in improving the clarity of the image.
  • Weatherproofing: There are two types of designations commonly used when talking about weatherproofing. The differences are: a weather-resistant optic is one that will withstand the occasional light shower but will not withstand submersion or extended use in wet conditions. weatherproof or waterproof optics means that the optics shouldn't allow water intrusion under most circumstances. Sometimes they are even rated to a depth of submersion for a period of time. Some manufacturers will even have an IP (International Protection) Rating. Binoculars with external focusing 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.
  • Armor is an outer coating of rubber or synthetic material. This protects the binoculars from scratches and corrosion. It can also make the binoculars easier to hold. A rubber coating does not make a pair of binoculars waterproof.
  • Gas Filled are binoculars that have been filled with a gas like nitrogen to prevent interior condensation. This can happen when a pair of binoculars is moved from a warm environment to a cold environment, or vice versa. This causes condensation inside the internal optics, fogging them up. Mold can also form over time from repeated condensation. Most weatherproofed optics are gas filled, but not always.


When choosing a pair of binoculars you are faced with making compromises. Binoculars that you use for astronomy are not going to be compact and you probably won't take them hiking. Below are some things to consider based on use:


For hiking, backpacking, and traveling, size and weight are probably going to be your main concern. A roof prism or a reverse porro prism will be more compact. Look for a pair with an objective lens between 32mm to 20mm. Most compact models have a magnification of 6x, 7x, 8x, 10x. 7x and 8x give a nice balance between magnification and a wide field of view. Suggested size: 8X25.


Size and weight are a consideration for wildlife viewing, but don't compromise too much. Look for an objective lens between 35mm and 42mm. You can use a pair of binoculars with an objective lens of 50 but it will add weight. For magnification stick to 7x, 8x, or 10x. Lower magnification will give you a wider field of view and higher magnification will provide more detail. If your primary use will be in big open spaces, go for more magnification like a pair of 10x42. For densely wooded areas go for lower magnification and a wider field of view like 8x42. 8x42 is the most popular birding and wildlife viewing binocular size.


When out on the water, a rugged waterproof design is essential. Low magnification will make it easier to see on a pitching boat. 7x to 8x magnification should do just fine. You will also want a larger objective lens. Look for an objective lens that is equal to or greater than 35mm. Suggested size: 7X50.


When sky viewing at night, a large objective lens is going to help you gather more light. Increased magnification with the same aperture will darken the background. This allows the stars to really stand out. Lower magnification will give you a wider field of view. You might also want to consider a pair of binoculars that can be mounted on a tripod. Look for an objective lens that is 42mm or greater. Magnification for handheld binoculars should be between 8X to 12x. Any larger and you will need a tripod or monopod. Suggested size: 10X50.


Just remember the best pair of binoculars are the ones you use. If they are comfortable and work for what you want them too, then they are the right pair of binoculars. If you are thinking about upgrading your current pair, please consider donating your old pair. The Birders' Exchange supports bird watching programs and research in South America. You can always give your old pair to them.