Binoculars are two identical telescopes ganged together so that a person can look through them with both eyes simultaneously, providing a three-dimensional image similar to what would be seen without the use of binoculars. Using both eyes is also more comfortable and more natural compared to using a telescope, for which one eye has to be closed to see properly.
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 brand, the quality of the optics, the types of coatings applied to the lenses and other features that might be added.
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 pair of binoculars is determined by the type of prism they use. There are two types of prisms: roof and porro. 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, providing greater depth of field and a wider field of view, but at the expense of a bigger, heavier design.
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 flopped.
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 dictates the thickness of the binoculars, or the diameter of each cylinder, and how much light is able to pass through them. If you know anything about camera lenses, you know that the larger the maximum aperture, the more light gets through them, which allows the camera to take acceptable pictures in low-light situations. It’s the same with binoculars; the larger the objective lens, the more light gets through and the brighter and more detailed the image will be.
Like a zoom lens, zooming binoculars offer variable magnification. Binoculars designated as 10-30x60 offer 10x magnification at the low end and 30x magnification at the high end. There’s usually a thumb lever or wheel that lets you adjust the magnification level.
The objective diameter also determines the exit pupil diameter, which should always be larger than the pupil of your eye. The pupil of a human eye ranges from about 1mm in bright conditions to about 7mm 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.
A binoculars’ exit pupil diameter is determined by dividing the second number by the first. So a 10x42 pair of binoculars 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, 10-30x60 binoculars have 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.
Many binoculars have no weatherproofing, while some are waterproof and others are waterproof and fog proof. Binoculars that have no weatherproofing should not be used in the rain or at sea, as moisture can get inside them, rendering them useless.
Waterproof binoculars are sealed with O-rings to prevent moisture from getting inside; but they can still fog up on you. Fog-proof binoculars are air- and water-tight, and filled with pressurized nitrogen gas to prevent fogging. Not all waterproof binoculars are fog proof, but all fog proof binoculars are waterproof.
The terms “angle of view” and “field of view” are almost interchangeable. Both terms describe the amount of scenery, measured horizontally, that is visible when looking through a particular pair of binoculars. 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.
Unlike an ice pack for a black eye, eye relief for binoculars takes the form of eyecups that block extraneous light and eliminate glare, and also keep the viewer’s eyes a specific distance from the eyepieces. If you’re wearing glasses, the glasses will hold the eyepieces away from your eyes so that you no longer can use the eyecups.
Simple eyecups fold away from the eyepieces when needed, and fold back snugly against them when not needed. Fancier binoculars will have adjustable eyecups that twist in and out to set the proper distance for the individual user.
Eye relief does not come without a cost, and this is the field of view. You’ll generally find that those models with more eye relief have a smaller field of view at 1000 yards than similarly priced models with less 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.
People’s eyes do not possess identical vision. Sometimes one eye sees better than the other. Dioptric correction is an adjustment that lets you obtain precise focus for both eyes. Binoculars with dioptric correction will have an adjustment on one of the eyepieces that is used to compensate for differences between the two eyes. Basically you set the dioptric adjustment to neutral, or center, and use the main focus wheel to adjust for perfect focus in the eyepiece that doesn’t have dioptric correction. Then you adjust the dioptric eyepiece so that eye also sees a properly focused image.
Rangefinder binoculars contain battery-powered circuitry and a laser that’s used to measure distance from the binoculars to the object that’s in focus. They can be used at sea to measure the distance to another ship, or possibly, someone that 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.
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 a helicopter, thus presenting the viewer with a steadier 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. In general, the larger the exit pupil diameter, the less important stabilization becomes.
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.
Lens coatings, as the term implies, are coatings applied to lenses to eliminate glare and reflections, increase contrast and make colors look more vivid. Any light reflected is light that never reaches the viewer’s eyes, so by eliminating reflections, you also end up brightening the image.
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 out 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.
It’s a good bet, too, that $50 binoculars will be worth about that much, no matter how impressive they look. A pair that costs $1,000 or $2,000 will likely perform many times better than inexpensive ones. But inexpensive is definitely much better than none at all.