Binoculars Buying Guide
Many consumers feel daunted when faced with the overwhelming options available in the binoculars market. It is always best to shop for a binocular in person, or at least speak with a knowledgeable professional to help you reach a decision, but buying from the internet can be made much simpler if you follow these guidelines:
1) Settle on a combination of prism type, magnification & objective diameter.
The prism is the device that shows the image to the observer as rightside up and laterally correct, because lenses project their images of the world upside down and backwards. The are two types available: "roof " & "porro".
Porro prisms consist of elements of glass offset from one another at an angle, while the glass in roof prisms are on top of one another.
There are benefits to both designs: roof prism binoculars are a more streamlined shape, capable of being held by those with small hands a bit easier than porro prism models. Porro prisms often offer a longer depth of field (the area of acceptable focus in front of and behind the actual focus point), a wider field of view and a meatier shape for those with larger hands. Both photos show binoculars of identical magnification and objective diameters, but the difference of prisms makes for remarkably dissimilar shapes.
Choose a sensible magnification for your intended use for the binocular. Novice buyers typically are attracted to higher magnification models (the first number in the binocular description refers to the magnification power, i.e. an “8x20” model will make things look 8x closer than your naked eye), but these higher magnifications don’t come for free!
Buying a binocular is always a compromise of size, weight, eye relief & field of view, no matter what your budget may be. So if you’re using it for the theatre, don’t pick anything more powerful than a 7x. Typical all-around binocular use is best kept to a 8, 9 or 10x power-the higher the power the more stunning the views and the more difficulty you’ll have holding the binocular steady.
Additionally, higher magnification models need subsequently larger objective diameters to remain bright-and that means a bigger, heavier rig. Models greater than 10x should be used for those with the specific need, such as big game hunters, police observation/surveillance, etc. These are quite difficult to handhold and retain a steady image. A tripod adapter is recommended for using these models, but certainly makes extended enjoyment of any binocular a less daunting experience.
Choosing an objective diameter directly relates to how much girth you’re willing to tolerate and in what kind of lighting you desire to use your binocular. Dividing the second number by the first in a binocular’s description tells you the size of the exit pupil diameter (for example, an “8x42” has a 5.25mm exit pupil because 42/8=5.25). This is a very important statistic. If the pupil of your eye is broader than the exit pupil diameter while you’re using the binocular, you certainly won’t enjoy the view very much!
Typically a human eye rarely gets wider than 7mm, and that is at the limits of light-so a model with an exit pupil diameter greater than 7mm (for example a “7x50”) can be used in all but total darkness. During twilight or dawn/dusk conditions you must have an exit pupil of 5mm or more to enjoy the view. And in bright conditions your pupil can be as narrow as 1mm or smaller, so models such as “8x20” are limited to effective use in bright conditions.
There’s one final consideration in this choice, and that is that if you’ll be using the binoculars from an unsteady observation point (such as a boat), you will need a model with an exit pupil diameter greater than your pupil’s diameter while in use. This compensates for the erratic movement of your pupil relative to the exit pupil of the binocular, and explains why boating binoculars have such large objectives despite often being used in favorably bright lighting conditions. Stabilized binoculars can also help in this regard, but are still limited by the constraints of exit pupil diameter.
2) Decide what level of weatherproofing you’ll need.
There are essentially only three relevant levels of weatherproofing: none at all, solely waterproof & waterproof and fogproof. Models that are both waterproof & fogproof have been sealed with O-rings and injected with high-pressure nitrogen gas during their construction. The O-rings prevent water from permeating the optic, and the gas keeps the internal temperature consistent due to the higher pressure within the binocular than the air around it. If you’ll just be using your binocular in benign environments of a friendly tourist, it isn’t necessary to spend the money on weatherproofing if your budget is restrained.
Waterproof and Fogproof?
If you’ll be using your binocular in situations where it may get splashed a bit but won’t be subjected to extreme temperature and humidity changes, a waterproof-only model will do. If, however, you expect to use your binocular successfully after immediately taking it through broad temperature changes such as leaving a heated room for winter air or entering humid Florida summer heat from an air-conditioned hotel room, you will need a binocular that is both waterproof and fogproof. Unfortunately there are less options with brilliant optics that don’t have weatherproofing, so those that just want great glass are often left with having to foot the bill for other features not necessarily needed.
3) Remember that more eye relief means a narrower field of view, and vice versa.
What’s Eye Relief?
If you wish to use your binocular while wearing eyeglasses or sunglasses, you’ll subsequently need to choose an optic that provides adequate eye relief. Those with thick glasses will likely need at least 15mm of eye relief to be able to still see the entire image created by the binocular. Eye relief adds to the comfort level of all observers, with or without the use of glasses, because you don’t have to constantly press against the optic to see the view.
Why Sacrifice Any Eye Relief?
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 will 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, and that expense will of course be passed along to the consumer. It is always nice to have a broad viewing area, so decide how much eye relief is necessary for you and buy the binocular that otherwise gives the widest field of view.
4) Binoculars with highly coated lenses and smaller objectives can appear brighter than models with less coatings and larger objectives.
The Semantics of Marketing
There is a lot of slippery language in the manufacturer’s descriptions of how much coatings they employ in construction of a binocular. Just like everything else, it is best to approach things with a jaundiced eye when comparing two models with similar specifications but incredibly diverse prices.
Coating lenses again and again to increase the amount of light that reaches your eye results in brilliant images, but also adds to the manufacturing costs significantly. A binocular can have an enormous exit pupil but still have a drab image, and a binocular can have a narrow exit pupil but nonetheless astonish you with its performance.
Following these tips should help you pick the binoculars that fit your budget and needs, so get out there and enjoy the wonderful world of “glassing”!
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