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Binoculars are two identical optical devices that use a series of lenses, elements, and prisms to magnify a view for more detail of distant people, places, or things. Observing with both eyes open is more comfortable and more natural compared to using a spotting scope or telescope, for which one eye has to be closed to see properly. Having both eyes open also provides you with a rich and immersive experience where the scene takes on a more life-like, 3-D appearance.
If you’ve been studying the design of different binoculars, you might have noticed that some look very streamlined while others look chunkier and heavier. That’s because the physical appearance and size of a binocular is determined by the type of prism it uses. There are two types of prisms: roof and porro. Prisms are used to present an image as right-side up and laterally correct; without a prism, binoculars would make things look upside down and reversed. The glass elements in a roof prism are in line with one another, making roof-prism binoculars more streamlined and easier to hold. Porro prisms have the glass elements offset from one another, and can provide greater depth of field and a wider field of view when compared to similar roof prism models. This is accomplished by folding the light path, which shortens the length, possibly spreading the objectives further apart.
There can be a huge range in price between apparently similar pairs of binoculars. For example, B&H sells 10x42 binoculars ranging in price from $30 to $2,500. The main reasons for such a huge price range are the quality of the optics, the types of coatings applied to the lenses, and other features that might be added, such as the material that comprises the housing. Additionally, the prism type can be a factor in determining price. Because of the physics involved in designing and manufacturing the compact roof prism form factor, you can have a pair of roof and porro binoculars that seem identical as far as quality and performance, but the roof prism version will often be more expensive. The good news is that if the form factor isn’t an issue, many people find that they can upgrade the quality of their binocular by choosing a porro-prism without reëvaluating their budget.
Technically, the type of prism utilized in binoculars is a double-Porro prism, but is always shortened to just Porro. It is also always capitalized because it is the last name of the inventor. This most basic of prism configurations is defined by the folded light path which displaces the point where the light enters and exits the prism, which results in the familiar and ubiquitous look of a binocular.
The term “roof prism” was originally applied to the Abbe-Koenig (AK) prism design that corrected an image horizontally and vertically while maintaining a straight line from the point the light enters the prism and exits it. While the AK configuration is the most common, there are others—most notably the Amici and Schmidt-Pechan (SP) designs. While they accomplish the same basic function, the optical paths take different routes to correct the image orientation. The main advantage of the SP design is that it is more compact than both the Amici and AK prisms.
It’s a Numbers Game
Magnification and Objective
Binoculars use numbers such as 10x42 and 7x20 to indicate their magnification power. Using 10x42 as an example, the 10x means that the binoculars offer 10x magnification power; in other words, things appear 10 times closer than they do with the naked eye. Everyday binoculars should be between 7x and 10x power. Theatergoers should choose 7x while big-game hunters would need 10x or higher. Keep in mind that anything higher than 10x is very difficult to hold steady without a tripod.
The 42 in our 10x42 binoculars refers to the diameter of the objective lens. It also affects the size of the binoculars and how much light is able to pass through them. In basic terms: larger objectives allow more light to pass through them versus smaller lenses, which means images will appear brighter and clearer. Zoom binoculars offer variable magnification and are shown as 10-30x60. In this example, 10x magnification is at the low end and 30x magnification at the high end. There’s typically a thumb lever or wheel placed conveniently within reach while holding the binocular so you can adjust the magnification without changing your grip or taking your eyes away from the eyepieces.
The exit pupil is the size of the focused light that hits the eye. To see the exit pupil, hold the binocular eight to ten inches away from your face and notice the small dots of light in the center of the eyepieces. Exit pupil diameter, which should always be larger than the pupil of your eye, is directly affected by the objective diameter and the magnification. The pupil of a human eye ranges from about 1.5mm in bright conditions to about 8mm in the dark. If your binoculars’ exit pupil diameter is smaller than the pupil of your eye, it’s going to seem like you’re looking through a peep hole. Bear in mind that as eyes age, they tend to dilate less, so exit pupil becomes more important as the user ages.
A binoculars’ exit pupil diameter is determined by dividing the objective by the magnification: so a 10x42 binocular has a 4.2mm exit pupil diameter. That’s a generous size, and larger than the pupil of the eye most of the time. But a 10x25 pair of binoculars has just a 2.5 mm exit pupil diameter, which is smaller than the pupil of the eye most of the time and will be harder to see through clearly.
Zooming binoculars might have a perfectly acceptable exit pupil diameter under low magnification but one that’s somewhat small under high magnification. For example, a 10-30x60 binocular has 10x magnification at the low end and 30x magnification at the high end. At 10x, the exit pupil diameter is a respectable 6mm, but at 30x it’s only 2mm.
Eye relief is the optimal distance from the eyepiece to your eye, or the focal point where the light passes through the ocular lens. Manufacturers install eyecups on the eyepieces to place the user’s eyes at the proper distance from the eyepieces to make using them easy. If you wear glasses, the lenses will position the eyepieces past the eye relief distance, affecting the image quality. Many binoculars offer dioptric adjustments on one of the eyepieces so that most users can finely tune the focus mechanism to their optical prescriptions so they can use the binocular without their glasses. If your prescription is difficult, or you’re sharing them with other users, the eyecups are often adjustable. Basic eyecups simply fold back to allow you to place your eye glass lenses closer to the ocular lens. Another type is adjustable eyecups that twist in and out to set the proper distance for the individual user.
You’ll generally find that models with longer eye relief have a smaller field of view than similarly priced models with shorter eye relief. Accomplishing superlative statistics in both categories is an expensive process of optical engineering. It is always good to have a broad viewing area, so decide how much eye relief is necessary for you and buy the binoculars that otherwise give the widest field of view. Field of view is discussed in greater detail below.
Glass, Prisms, and Coatings
The type and quality of the glass used for the lenses and prisms matter. Generic optical glass may have imperfections and if they aren’t ground and polished correctly they could bend light oddly, causing colors to look off or prevent fine focusing, or you may notice distortion at the edges. Specialized glass, such as low dispersion or extra low dispersion, is engineered to have virtually no distortion and transmit light better without bending it. The resulting images are generally clearer, sharper, with true color rendition and higher contrast.
You may also see a category called “Eco-glass.” This general term refers to ecologically friendly glass that doesn’t use lead or arsenic while it’s being made. While this may or may not affect the image quality, if your lenses break or you need to dispose of your binocular, you can feel confident that you’re not adding to chemical pollution.
BAK4, BK7, and SK15 Prisms
BAK4, or Barium Crown glass, is considered the best type of prism material. It has a high refractive index and lower critical angle than other materials, which means it transmits light better with less peripheral light being lost due to internal reflection—for example, as from internal bubbles during the manufacturing process.
BK7 glass is arguably the most widely used material for binoculars. While it may be considered to be of lower quality than BAK4, it is still optical glass, which means it has excellent light-transmission properties and a limited number of internal imperfections.
The easiest way to tell if your binocular employs BAK4 or BK7 is to turn it around, hold it 6-8" from you and look down the objective. If you can see a squared-off side to the general roundness of the image, the binoculars have BK7 prisms. BAK4 prisms show a truer round, which translates to better light transmission and edge-to-edge sharpness.
SK15 glass is an atypical material that strikes a middle ground between the previous two. It has a higher refractive index than both, yet has a dispersion (measured on the Abbe scale) that falls between BAK4 and BK7. Images that are seen through SK15 prisms are very clear, with high contrast.
Lens coatings are sheer films applied to lens surfaces to reduce glare and reflections, increase light transmission and contrast, and help make colors look more vivid. Any light reflected is light that never reaches the viewer’s eyes, so by eliminating reflections, the image ends up being brighter. Coatings, in general, are good, provided that the coatings do something. It’s easy to put a cheap coating on a lens to give it a cool-looking orange tint, but the coating might not do anything to improve image quality. If you aren’t able to test a pair of binoculars before buying, the best you can do is research the brand, look for user reviews and ask questions before you buy.
Terms such as coated, multicoated and fully multicoated refer to the location and type of coating processes used. Coated lenses are the most basic and denote that at least one lens surface has at least one layer of coating on it. Multicoated means that multiple surfaces are coated and/or multiple layers of coatings have been applied to each surface. Fully multicoated means that all surfaces—inner and outer—of the lenses have multiple layers applied to them. This treatment allows the highest level of light transmission, clarity, contrast, and color rendition. At the pinnacle is broadband fully multicoated. These coatings are engineered to be effective across a wide spectrum of wavelengths and offer the best performance.
Complementing lens coatings are prism coatings. These increase light reflection and improve image brightness and contrast. While many manufacturers may use standard reflective coatings, the upper echelon of prism coatings is called dielectric coatings, which allow almost 100% of the light through the prism, resulting in brighter high-contrast images. Another type of coating “phase-corrects” the light. When light passes through the prisms, the different wavelengths move at different speeds through the prisms, affecting the color and contrast. Phase-corrected prisms correct this and ensure the light comes through at a uniform speed, which greatly impacts the image.
Angle of View and Field of View
The terms “angle of view” and “field of view” are complementary. Both terms describe the amount of scenery, measured horizontally, that is visible when looking through a particular binocular. Angle of view is expressed in degrees. Imagine standing in the middle of a giant pizza pie; binoculars with a 6.3-degree angle of view would show the viewer a 6.3-degree “slice” of the 360-degree pie, looking outward.
Field of view is expressed in feet, at a distance of 1,000 yards. For example, say you’re looking at a ship that’s 1,000 yards away; binoculars with a 330-foot field of view would show you 330 feet of the ship, measured horizontally.
Minimum Focus Distance
This might seem like an odd thing to consider, since the whole idea of a binocular is to look at things from afar; and for most users that is absolutely true. However, there are a fair number of enthusiasts that use their binocular for bird watching or insect observation. Many bird watchers like to have a fairly close minimum focus distance that can allow them to see minute detail of birds—foot or beak markings, feather size, etc.—while birds are feeding. A close focus of less than 6' for a full-size binocular is note worthy. Typically, as magnification is increased, the minimum focus distance also increases. For users looking for a short close focus distance, emphasize objective size and keep the magnification at around 8x.
This is sort of a catch-all category to discuss some design features that speak to the form and function of the optic, rather than the performance.
Open or Closed bridge refers to the center portion that connects the two optical tubes on roof prism binoculars. Typically, the center hinge and focusing mechanism will be enclosed in the housing material. While this strengthens the hinge and mechanism, the closed bridge prevents your hands from wrapping all the way around. An open bridge will usually have the focus mechanism close to the eyepieces and another stabilizing section toward the objectives, with the middle section left open. This not only enables a full wrap-around grip, but it also cuts the overall weight of the optic.
The vast majority of binoculars use a center focus system. The main focus wheel is set between the two oculars and moves them symmetrically. With center focusing, many manufacturers will have a dioptric adjustment dial on one of the eyepieces to fine-tune the focus to match individual optical prescriptions. The dioptric correction amount is decided by each manufacturer, usually by model, and can be on the left or right eye – or both. Certain models have the dioptric correction integrated into the center focusing mechanism.
There are two other focusing types that need to be addressed: individual and focus-free. Individual focus is when each eyepiece is individually focused, allowing for extremely fine and precise focusing. Many marine and astronomical models feature this system. Focus-free binoculars don’t have any focusing mechanisms. They rely on your eyes to focus the image, allowing you to concentrate on the scenery and enjoy the views.
Weather-Resistant, Waterproof, Fogproof
Many binoculars have no weatherproofing, while some are waterproof and others are waterproof and fogproof. The rating will determine under what conditions the optic should or can be used.
No rating: Binoculars that have no weatherproofing should not be used in the rain or at sea, as moisture can get inside them, rendering them useless.
Weather-resistant: Often, but not always, they will employ some kind of seal—an O-ring or gasket—to keep moisture, such as from a light mist, from getting into the optical tubes.
Waterproof: These binoculars are sealed with O-rings to prevent moisture from getting inside; but they can still fog up on you. Depending on the construction and the seals, some waterproof binoculars are also submersible to certain depths for various amounts of time. Certain manufacturers rate their binoculars for limited depths for limited amounts of time; others will adhere to military standard specifications and rate them for much greater depths.
Fogproof: Fogging occurs when the air inside the optical tubes contains moisture. If you go from a warm cabin outside in the cold, the moisture can condense on the inside of lenses, causing them to fog. Fogproof binoculars are filled with inert gasses such as nitrogen or argon, or a combination of the two, to prevent fogging. The inert gas is dry and is pumped into the optical tubes under pressure, keeping the gaskets and O-rings firmly in place.
The Takeaway: Not all waterproof binoculars are fogproof, but all fogproof binoculars are waterproof.
Rangefinder binoculars contain circuitry and a laser that’s used to measure distance from the binocular to the object that’s in focus. They can be used at sea to measure the distance to another ship, or possibly, someone who needs rescuing. Or they might be used by game hunters to measure the distance to their prey. They can help golfers calculate their swing from the sand trap to the next hole. Rangefinder binoculars typically display the distance to the target in either feet or meters, with the readout visible in the eyepieces.
Technological innovations have made the rangefinders more precise and some have the ability to do a single spot measurement, or a constantly updated measurement so you can follow a moving subject and get virtually real-time distance. The latest versions will incorporate an inclinometer that measures the uphill or downhill angle from you to the subject. Golf or hunting-specific versions may take the distance and angle, and calculate an adjusted distance for you to judge your shot. In the future, you can expect to see GPS tracking or location logging being further integrated into these types of binoculars.
In the same way that digital cameras can have image stabilization, so too, can binoculars. Image stabilization compensates for operator movement, the swaying of a boat, or the vibration inside an aircraft, that normally prevent the viewer from having a steady image. Stabilized binoculars might contain a gyro that requires power to provide stabilization, or perhaps a pendulum-type device that provides stabilization without being powered. Most often, this type of binocular is used by boaters to reduce the disorientation common with high-power optics, or while using them in choppy seas. They are also popular with aviators and search-and-rescue professionals.
These specialty binoculars will have features and functions that are made specifically for certain jobs or environments. Marine binoculars will often have polycarbonate housings that are corrosion and temperature resistant for use in salt-water environments, buoyant so if they get dropped overboard, they can be easily retrieved; others still will feature bright colors to make them easier to spot.
Some binoculars can contain integrated compasses. The two main types are digital and analog. They will have a scale somewhere in the field of view to show the current direction. Digital compasses will be battery powered and illuminated for use in most light conditions. Analog models will use batteries or might have an opaque-covered window on the top of the housing to channel and focus ambient light to illuminate the compass bearing. Many marine, image-stabilized, and rangefinder models will offer versions with or without compasses.
Basic accessories serve to replace lost or broken stock items or can simply make carrying or using your binocular a bit easier. These easy upgrades can include:
Tethered caps: These will have a ring that loops over the objective end of the housing so when you need to take the caps off you just flip them down and you don’t have to worry about losing them.
Rain Guards: Replace your stock eyepiece caps with a one-piece rain guard that prevents the eyecups from flooding. It will often attach to the neck strap to keep it safe and handy for flash showers.
Straps: Not satisfied with the thin nylon strap that came with your binocular? Get a new one that’s longer, adjustable, padded, ergonomic, buoyant, colored.
Cleaning kits/supplies: Solutions, pens, cloths, cleaners, kits—everything and anything you need to clean and maintain your optic properly.
As mentioned before, binoculars that are 10x and higher are hard to hold steady, especially if they are big and heavy. Large binoculars should have a built-in tripod mount that makes it easy to mount them on a tripod. Sometimes a tripod adapter is required. Typically, full-sized binoculars have a plug that unscrews from the front of center hinge. The adapter screws in its place and mounts on most quick-release plates or tripods. Some tripod mounts are simply a small platform to lay the binocular on and hold it in place with an adjustable strap.
For most of us, the neck strap that comes with most binoculars is fine. For those who require more, there are numerous options for you. Some are designed to redistribute the weight of the binocular from the neck to the back and shoulders. Others provide a stabilizing function to allow you to hold the optic in your hand while virtually eliminating hand shake or other movements. For those who do activities and want to keep their optic at the ready, some harnesses hold the binocular close to the body and greatly reduce swinging or swaying while running, climbing, or skiing.
As we discussed earlier, the eyecups hold the eye at the proper distance from ocular lens. Some manufacturers offer eyecup upgrades for certain models. The most popular are replacing standard flat eyecups with winged eyecups. The wing wraps around and blocks your peripheral vision, which eliminates light leakage for improved image brightness a clearer view.
The use of digiscoping adapters has seen an increase in recent years, since just about every phone in everyone’s pocket is equipped with a camera. These adapters, often binocular and phone-specific, allow you to mount your phone on one of the eyepieces and take photos of the magnified view. Depending on the manufacturer, these adapters can be made of plastic or metal with varying degrees of usability options. The good news is that as the hobby grows, more and more options are made available so you can spend as much or as little you want.
The world of binoculars is vast and constantly evolving. No matter what you’re using them for—from a night at the opera to hunting on the tundra to comet watching—there is something for everyone at every price. This article has offered a basic introduction to the terms and technologies that will affect your buying decision and the overall performance of the optic. After making your selection, don’t forget about the accessories that can enhance your viewing experience and turn a good view into a great view.