How Binoculars Work & FAQ

It’s easy to get overwhelmed by all the features, functions, and options available when considering binoculars and other devices such as monoculars and scopes. With so many tools out there, how do you choose the one that is best for your use? How do binoculars work, and why do they range so much in price? What are all those numbers on the box, and what does FOV stand for again? Don’t get lost in sales speak about eye relief and coated lenses – we’ve got all the explanations here for you.

Binoculars come with a wide range of features. Each uses lenses and prisms in slightly different ways, and choosing carefully can mean the difference between an excellent experience and a blurry image.

Galilean binoculars are the simplest, and each use four lenses – two convex and two concave. Although these lenses automatically provide an upright image, only low magnification is possible, along with a very narrow view. Therefore this design is only used for inexpensive binoculars along with opera and theater glasses.

More modern binoculars are designed using prisms in conjunction with lenses. In this case the lenses offer a better field of view and higher magnification, but result in an upside down image. Consequently prisms are used to set the image upright again for viewing.

Porro prism binoculars are created by placing prisms in a double Z-shape. This prism placement requires more area, resulting in wider binoculars, and allowing for bigger lenses and thus, better light collection. After time the prisms may need to be re-calibrated to ensure they work effectively, however this design is less expensive than Roof prism binoculars, and more popular.

Roof prism binoculars are created by using shallow, small prisms and result in more compact binoculars. Their smaller size makes them a more durable option for hunting or as a spare pair to stay in your glove compartment. They can also be lighter in weight and easier to hold for long periods. Consider them when optimum quality is needed along with easy carrying.

Roof prisms are collimated more tightly and securely when they are produced, so they tend to take rougher handling better without the need to be recalibrated. However, these prisms require special coatings to use light effectively, including phase coating, aluminum coating, silver coating, and dielectric coating.

What do all those numbers mean?

Ultimately Porro prisms and Roof prisms can offer comparable images. In fact, the following details define quality for both types of binoculars.

Magnification and objective lens diameter

These are always described together using a pair of numbers such as “7×50”. When described this way, the first number refers to the magnification amount, and the second to the diameter of the objective lens. For example, “7×50” means that images are magnified 7x, and the objective lenses have a diameter of 50mm. These measurements are of greatest importance to image quality so need to be considered carefully.

Higher magnification will allow you to see objects further away, but will narrow your field of view. So you’ll see further, but for objects close to you, you’ll be restricted to looking at a smaller area and refocusing if that object is moving. A lower magnification from about 7x to 10x is fine most of the time, whereas magnifications of 20x to 30x are necessary forastronomy.

Larger lens diameter allows more light to enter the binoculars and reach your eye. While they increase weight significantly, bigger lenses are best for night-time viewing, hunting at dusk or dawn when game move, and even low-light environments such as heavily wooded forests.

How do I choose the right magnification?

Magnification is shown on binoculars as the first number in labels such as 7×50, 8×25, and 10×42. If you see two numbers separated with a dash, that means the binoculars zoom (ex. 7-25×42). If you see two numbers separated by a slash, that means there are different eyepieces to allow different magnifications (ex. 7/25×42).

Higher magnifications let you see further, but reduce how much area you can see through the binoculars at one time. Therefore lower magnifications are better for some uses.

  • Magnifications of 2x to 5x are great for monoculars.
  • Magnifications of 7x to 10x are perfect for most uses, including hiking, hunting, andbirdwatching.
  • Spotting scopes will have higher magnifications around 20x.
  • For astronomy binoculars, magnifications of around 10x are fine. You may wish to look for telescopes or binoculars with magnification from 20x to 30x and up though. Some telescopes have magnifications as high as 160x and interchangeable eyepieces so you can use different magnifications, but often the larger options also need to be mounted on a tripod.

How do I choose the right lens size?

Objective lenses collect more light as they get larger, making binoculars and other optical devices effective even in lower light. That makes them more effective when skies are gray, the air is foggy, or at dawn and dusk. However, larger lenses are both more expensive and heavier, so the largest ones can require mounts or tripods to be used. 20 mm lenses are often fine for birdwatching and hunting during the day, while 60 mm, 80 mm, and even 100 mm and larger lenses are best for stargazing.

Should I choose binoculars with Porro Prism or Roof Prism?

Porro prism binoculars are designed using prisms in a Z-shape. The prisms themselves encompass a wider area, so the objective lenses are spaced further apart. This type is usually more bulky but often perform better at a lower price. They yield a better three dimensional and wider field of view, but may be more likely to be knocked out of collimation when bumped or dropped. Roof prism binoculars are more streamlined, compact, and lightweight, but must have anti-reflective coatings to work effectively. They are more durable, less likely to fog up or allow dust into inner mechanisms, and their internal focus system gives them better integrity as well.

You may wish to choose Porro prisms when you want to start with a lower priced model that still performs well. If you are hunting or using binoculars on a boat or in the military, the extra investment in tougher, lighter roof prism binoculars is worth it. Ultimately either type can give clear images and offer good magnification and light collection, as these are more a result of objective lens size and all the mechanisms working together rather than one prism design or another.

The Power of Porros

Although not as popular as they used to be the Porro prism binocular still continues to provide many advantages over the now common roof prism binocular. Porro binoculars offer a superior depth of field and stunning contrast yet there popularity is still on the wane. Find out why this classic design is still one of the best and why they still pack a big punch when it comes to price versus performance.

Take a casual look at the binoculars for sale most anywhere today, and you will notice one striking pattern—they are predominantly roof prism models. Not originally well received when introduced to the market around a century ago, roof prism binoculars have made tremendous gains in popularity over the past two decades as consumers have come to prefer the slimmer, smaller profile the roof prism design makes possible.

But while it is now somewhat out of favor, the roof prism’s predecessor, the Porro prism, offers particular features and benefits that roof prism models are not as good at providing. For this reason, anyone considering buying new binoculars would do well to add a few Porro prism models into their potential choices.

The Porro prism binocular—named for its inventor, Ignazio Porro—was the first major improvement upon the original Galilean binocular design, which was essentially two telescopes connected together with a hinged bracket. Not a poor design as long as the magnification levels were kept fairly low, the Galilean design suffered from the problem of needing to be made longer (to provide a lengthened optical path for the higher level of magnification) and larger (to increase the overall amount of light needed for that lengthened optical path) if a higher level of magnification was required. Without modern lens design technology or lightweight synthetic materials available for the chassis and other mechanical parts, a larger binocular meant a much heavier one as well.

Porro’s solution to this problem was ingeniously simple: use prisms to bend the light in the optical path of each half of the binocular so that a longer optical path could be contained in a more compact physical structure. Two identical prisms of a simple right triangle shape were used in each half of the binocular. These prisms were positioned midway between the objective and eyepiece lenses, perpendicular to one another and with their longer sides (hypotenuses) facing together at one corner. The Porro prism binocular design bends the light path of each optical channel in the binocular five times, greatly lengthening the distance the light travels without requiring the binocular itself to be made physically longer.

By using triangular prisms and positioning them in such a way, the objective lenses were no longer directly in front of the eyepiece lenses. In the classic Porro shape, the objective lenses are further apart from one another than the eyepiece lenses (which must be kept within a distance that is adjustable to the same distance apart as a person’s eyes). Thus, while the overall length of the Porro prism binocular was reasonably easy to handle and balance, the width was increased to accommodate the Porro prism assemblies.

This greater width was not necessarily bad. By spacing the objective lenses wider than human eyes, the visual image transmitted through the binocular to the user is one with greater depth of field, and of superior contrast. Just as the eyes of a predator, such as a lion, are set closely together in order to concentrate all attention on whatever it selects as its prey, so the eyes of prey animals, such as a gazelle, are set widely apart on its head in order to take in a wider view of the world in order to keep a sharp lookout for predators. Humans, being predators, have forward looking eyes that allow us improved perception of movement and focused vision, but rob us of the more three-dimensional view of the world that enables prey animals to discern the shapes of potentially dangerous predators, even when those predators are motionless. Likely without intending to do so, Porro solved not one but two problems with his innovative binocular design—make a more powerful binocular shorter, and enable that binocular to provide its user with an enhanced three-dimensional image of the object being viewed.

Sadly, the popularity of the roof prism has left all too many classic Porro prism binoculars neglected. In fact, just this year Nikon removed one of its most famous Porro models, the Superior E, from its product line. Fortunately, some fine Porro prism binoculars are still on the market. In fact, one of the finest Porro prism designs ever made is presently available from one of the optical world’s great names: Swarovski. The Swarovski Habicht 7x42mm, 8x30mm, and 10x40mm binoculars combine the classic European binocular design with state-of-the-art optical quality to produce an optical instrument of superb quality, as well as a true work of art in its physical design. Providing the user with an exceptionally vivid, three-dimensional image, Swarovski Habicht binoculars can bring out the subtle details in objects that most other binoculars on the market today simply cannot.

Taking the power to produce a three-dimensional image to unprecedented levels, the Opticron HR WP 8x42mm and 10x42mm binoculars are designed with exceptionally wide spacing between the objective lenses for one of the widest image triangulations of any binocular design presently available. Thanks to some creative engineering, these binoculars’ comfortable and ergonomic design allows them to be well balanced and very easy to use.

Steiner offers lines of military, marine, and law enforcement binoculars, nearly every one of which is a Porro prism model to provide those users with the ability to see objects or people that are intentionally trying to remain hidden. The company also offers the innovative mid-sized 8x30mm Wildlife Pro CF, which features the benefits of Steiner’s renowned automatic focus system, as well as their new Ultra-Sharp focusing system for making critical refinements if needed.

There are times when the ability to focus on an object from a very short distance is critical, such as when observing wildflowers, butterflies, orbirdwatching in a thickly forested area where the line of sight is limited to a few yards off the trail. Traditional Porros, due to their widely spaced objective lenses, cannot allow very close distance observation without the image being divided. However, by flipping around the Porro prism assemblies inside the binoculars, designers have been able to create inverted Porro prism models that, thanks to their objective lenses being even closer together than in roof prism models, can provide astonishingly close focusing abilities.

Among the best of these ultra-close focusing binoculars is the Pentax Papilio. Available in 6.5x21mm and 8.5 x21mm designs, the Papilio incorporates a revolutionary convergent lens design that allows focusing on objects from as close as 18 inches. In addition to the remarkable Papilio, Pentax also offers a wide selection of other inverted Porro designs, such as the UCF R and UCF WP series models. While not capable of providing the close viewing opportunities of the Papilio, they make good travel, event, and pocket binoculars thanks to their compact size.

Another reliable inverted Porro prism binocular designed with a relatively close focusing distance is the Nikon Travelite EX. Available in 9x25mm, 10x25mm, and 12x25mm models, the Travelite EX binoculars offer a small, easy-to-carry binocular that, thanks to their higher magnification levels relative to their close focus distance capabilities, can also be used for short distance observation of butterflies, birds, or perhaps even museum displays. An interesting recent addition to the Nikon line modeled on the Travelite EX series is the Nikon Ecobin, a 10x25mm inverted Porro prism binocular that uses lead-free glass, non-chloride rubber, and no harmful inks or dyes in its construction, making it one of the most green binoculars ever brought to market.

Before dismissing the idea of a Porro prism model as your next pair of binoculars, consider whether it what it can provide would be applicable to your intended use. Would an enhanced three-dimensional image make what you are viewing in the field easier to locate or observe? Might you need an ultra-close focusing distance in order to get a better look at insects or plants? While the answers to these questions might lead you to choose a model that isn’t widely popular, the basic rule of binocular choice should always be observed: the best binoculars for you are the ones that best match your ability to use them, and what you intend to do with them.

What is Field of View? (FOV)

Refers to the area visible to you as you use your binoculars. The number given tells you how big an area you can see when viewing objects 1000 yards away. For example, a FOV of 372 ft. means you will see an area of 372 ft. 1000 yards away from you. Wide angle lenses are available to increase that FOV for game hunters and sports fanatics.

This is the minimum distance at which you must hold the eyepieces away from you. This is important especially for those who wear eyeglasses, so make sure to test this in store to find a set of binoculars that are comfortable for you to hold to your eyes.

What is eye relief?

Eye relief refers to the distance you must hold binoculars or scopes away from your eyes to use them. Larger eye reliefs mean you can use your binoculars while wearing glasses.

What is an exit pupil?

Is the virtual opening at the center of each lens through which light can move on its journey through the binoculars. Optimally this exit pupil needs to closely match your own pupil, as light that travels outside yours is lost. Therefore this becomes important when you use binoculars at night – your pupils dilate to take in more light, so a bigger exit pupil of about 7mm is ideal for astronomy. However, during the day your pupil will be about 2.5mm.

Calculate the exit pupil by dividing lens diameter by magnification – so binoculars at 7×50 have a lens pupil of about 7mm and are good at night. Binoculars at 20×50 not only offer more magnification but a smaller exit pupil of 2.5mm which is ideal for bright daylight use.

What is phase shift?

Phase shift is a problem that is unique to roof prism devices. The prisms within these devices can cause a shift in light rays, leading to low contrast and poor resolution. In short, poor light focus within the binoculars results in a dim, blurry image. That said, high quality roof prism devices will use special phase coatings throughout. The coatings force light beams into phase again, making a sharp image where fine details are visible.

What do BAK4 or BK-7 mean?

BAK4 and BK-7 are common types of glass used in prisms. Both are types of crown glass, but BAK4 has a slightly higher level of refraction than BK-7. BAK4 is more expensive but is considered the better option because it creates a smooth, rounded exit pupil and a sharper image.

How do I find an optical device that is comfortable to use?

Try to handle binoculars and scopes in store to get an idea of their weight, shells, eye cups, and so forth. Soft eye cups and focusing mechanisms are important when you use your device a lot. For carrying binoculars in hand, you will want a set that weighs up to about 2lb to 3lb. Otherwise you may need to choose a mount as well. Binoculars are often more comfortable to use since they have barrels rather than the one found on a scope or monocular.

Should I buy wide angle binoculars?

When you look through an optical device such as binoculars, you are limited to viewing a certain area at one time. This area is known as the field of view (FOV). Wide angle lenses allow for a larger FOV than seen in standard binoculars. They achieve this using lenses and prisms to take advantage of light across more degrees. While you may see a FOV of 350 feet at 1000 yards in regular sets, wide angle models could offer a FOV of 500 feet. This is particularly useful when you are surveying the sky or watching a race, and need to see over more area at a time.

What binoculars accessories are available?

There are many binocular accessories out there, from cases to straps, specialized mounts, comfortable eye cups, interchangeable eye pieces, cleaning kits, and so much more. When using your binoculars on the move it can be worthwhile to look for heavy duty harnesses and straps to help keep them comfortable to carry. If you’re planning to stargaze though, choosing a heavy duty mount with adjustable angles is a great idea.

Interchangeable eyepieces are often available for bino-telescopes and other scopes, and can allow you to view objects at different magnifications as needed. Since zooming is still considered imperfect and problematic, interchangeable eye pieces are a cost-effective and useful accessory to consider buying. Just make sure your binoculars and eye pieces will work together.

Lens cleaning kits are almost inevitably a great idea if you are using high-end optical devices such as telescopes or high powered binoculars. You don’t want to scratch up your lenses when you’ve invested in a good set of binoculars. You might also get one for stashing in your glove compartment or tent if you use binoculars while birdwatching, hiking, hunting, or on a boat.

What are coated lenses?

You may not realize it, but the cost of a pair of binoculars is greatly influenced by the type of coating on the lenses. But just what do lens coatings do? And how do you know what kind you need? In addition to your budget, that decision depends on your viewing habits, and what kind of binoculars you need. Read up on the basics of binocular lens coatings, and you’ll be better informed when you make your purchase.

Coatings are described as follows:

  • Coated – One or more surfaces are coated.
  • Fully Coated – All air-to-glass surfaces are coated but plastic lenses may not be.
  • Multi-Coated – One or more surfaces have been treated with multiple coats.
  • Fully Multi-Coated – All air-to-glass surfaces are treated with multiple coats.

Among all the topics discussed in regard to binoculars and spotting scopes, one of the most common is lens coatings. What is applied to lenses (and prisms), how it is applied, and what it accomplishes makes up a large portion of the claims sports optic companies make about their products. As a result, it is of great interest to their customers. The subject of lens coatings can get complex very quickly, and as a result, become confusing. An understanding of lens coating basics will benefit anyone contemplating the purchase of a new spotting scope or binoculars. But first, a few things about light must be understood.

When thinking about how light behaves when it comes into contact with the lens of a binocular or spotting scope, the concepts of reflection, refraction, and transmission are important to keep in mind. When a wave of visible light encounters a glass lens, it may be partially reflected, partially refracted, and the remainder of it may pass through to the opposite side. In an optical instrument with multiple lenses, and perhaps even prisms included in it, it isn’t hard to understand how difficult it is for optical engineers to create the right balance of lenses and prisms.

Every image is composed of visible light waves, and must pass from the outer surface of the objective lens all the way through to the opposite outer surface of the eyepiece lens, and then into the observer’s eye. It must also be sufficiently magnified to be useful to the viewer, and at the same time remain sharp, clear, and bright. When you consider the challenge, it’s just half a step short of miraculous that it can even be done at all.

When Galileo created his first telescope, it was likely around 3x in magnification, and yielded an image that was most likely quite blurry. If modern engineers had only the quartz crystal available to that ancient astronomer for lenses, and no lens coating technology to enhance its qualities, we would likely not have progressed much further than Galileo’s design. Fortunately, we now have a dizzying array of types of glass and other materials out of which lenses can be fashioned. We also have a host of different types of coatings that can be applied to them in order to produce a seemingly infinite number of qualities and effects.

Simply put, each substance out of which a lens or prism can be made has certain qualities that produce a specific effect upon a visible wave of light when coming into contact with it. Coatings can then be applied to finished lenses or prisms in order to enhance, diminish, or slightly alter these effects. Why not simply choose a different type of lens material that provides the desired effect? Because while a particular type of lens material may have the precise light management properties desired for a specific effect, that material may lack another needed quality such as durability, it may be too expensive to use, or it may simply be too difficult to obtain in the quantities needed. Coatings can be applied to a type of lens material that, in combination with the coating, produces the exact effect upon the light desired by the optical engineer.

Metallic coatings are most common to the average person’s experience. In their simplest form, they are what make common household mirrors possible. In binoculars and spotting scopes, metallic coatings may applied to glass or other materials in order to increase the amount of visible light reflected from it. This process is often used on one or more faces of a roof prism assembly—the auxiliary prisms generally—in order to cause the light not to pass out through the face of the prism, but to change direction and travel on to another prism surface. The most common substances used for this are aluminum, and silver, which is preferred because it reflects visible light more efficiently.

Another type of lens coating often used in creating binoculars and spotting scopes is multicoating. The term multicoating, like many other technical terms encountered in marketing materials, can be tricky. While its benefits and capabilities have been explained correctly by some, others have misused or overstated them. Multicoating is, as the name implies, multiple layers of a coating applied to a lens surface. But unlike metallic prism coatings, the purpose for this type of coating is to reduce reflection when visible light comes into contact with the lens so that more of the light passes through it. While multiple coating layers are often beneficial, in some circumstances, a single layer coating may be better for the specific purpose the lens in question serves. As with most elements of optical design, the key is to use the right coating on the right lens, which in turn is made of the right material and placed in the right position in the optical system for the desired effect on the light to be achieved.

Multicoating commonly uses such compounds as magnesium fluoride, or calcium fluoride. A number of other more exotic, closely-guarded substances are also employed. In addition to the reduction of reflection in lenses, multicoating can be used to alter the way light is refracted as it passes through the lens. Simply put, these coatings, in combination with the substance of the lens itself, can help reduce the amount of separation of the visible light waves comprising the image to prevent distortion of the color, loss of clarity, or lack of sharpness in the final image perceived by the viewer.

The type of coating that has recently generated the most press coverage, but which still confuses many people, is phase coating. Applied to the roof prism in binoculars or spotting scopes, phase coating helps keep the image carried in the wave of visible light from being altered or rendered “out of phase” when it leaves the prism assembly. Roof prisms are somewhat tricky. Developed to allow a long optical path to be contained in a small space, roof prisms have the potential to disrupt an image composed of visible light by splitting it and recomposing it with the longer wavelengths out of phase to the shorter wavelengths. Phase coating helps prevent this by changing the speed of one portion of the split light waves so they are “in phase” with the rest when exiting the prism.

In addition to metallic coatings to improve reflectivity, and anti-reflective multicoatings to reduce it, an assortment of new coatings have been developed recently that can help reduce dust or water drops from accumulating on lenses. An example of the first of these is the proprietary Swaroclean coating found on Swarovski binoculars and spotting scopes. With this coating applied to the lenses, the surface energy is reduced, and the lenses are thus less adhesive to dust, water spots, and other residue, which helps keep them cleaner, and makes them easier to clean when necessary.

Regarding water on lens surfaces, Zeiss coats the lenses of many of its products with LotuTec, a proprietary water repellent coating that causes rain or mist to form small droplets on the lens surface rather than sheeting. Small droplets have less surface adhesion than water sheets, and can be more easily shaken free of the lens, leaving it dry and clean.

There is a lot more to lens coatings, but the basics are all you need to help you make the best decision about what you need, and what to look for when purchasing binoculars or spotting scopes.