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How to Buy Binoculars: About Binocular and Optics Specs

Many visitors to Lake Tahoe bring their binoculars for viewing wildlife and the lake up close! A good set of binoculars allows for crystal clear up close viewing for fun or sport!

To help you choose the best binoculars for your particular purpose, we start out with general recommendations. Then, we go into more details as you read. So, if you don't want to delve into the details of optics and confusing specifics, you can stop reading at any time.

Binocular specs start out simple. The format is...

How much do they enlarge the IMAGE?

multiplied by

How big is the LENS?

 

Opticians have a technical way of saying the above.

Their terminology is...

magnification X objective

 

For example:

8 X 42

enlarges images 8 times and the lens is 42mm diameter.

 

Or...

8-24 X 25

will zoom from 8 to 24 times bigger

and the lens is 25mm dia.


How does this apply to binocular uses?

It depends on what you want to view...

Binoculars to keep in the car: In your vehicle, you probably would rather have a compact set of binoculars that you can fit into the console, door bin or glove box. Again, consider the distance you will want to view at. In my experience, I like to look at things far off like mountain tops, across a lake, boats on the lake or an airplane. That distance is so far off it's considered infinity. But, I might also want to see up on a building or bridge top, which is closer but still could be 500-1,000 yards, so again - infinity as far as binoculars are concerned. So, look for COMPACTS with as long a range as you think you might use. For long distance you want high magnification, and that is the first number on the left side of the "X" in binocular specs. 8X may be OK but 10X will give even more clarity at distance. But, if you're looking closer up, any general purpose compact binoculars may suit you and 8X would be fine for you.

How to remember this: For seeing far away, get a high number before the X: 10X40 is better than 8X40 in this case.


Birding and wildlife: 8x40, 8x42 or if you have a steady hold, can tolerate wobble or have a tripod 10x40, 10x42 and roof prism. (You can read more about roof prism binoculars below.) Again, if it's damp or rainy where you go birding, look for fogproof and perhaps even waterproof binoculars.

Star gazing and astronomy without a tripod: If you want to go without a tripod, binoculars are really only good for looking at sections of the sky, such as a star field or a constellation. (If you want to view a planet, you will need a tripod and a scope so skip to the next paragraph) .So, you actually don't need a lot of magnification and 7X, 8X or whatever will do. Don't go higher than 10X or the picture will become very wobbly because your muscle movements will make the view wobble.

With tripod, you can go for the higher magnifications like 12X through 16X. A zoom like 10-22X50 would be fine.

As far as lens size goes, get X50 or higher. The higher the better. But, remember you have to carry your binoculars and X60 is heavier than X50.

Out on the boat: For marine binoculars, you most definitely want waterproofing and fogproofing.

Here you're worried about the wobble thing, because the boat bobs around in the water and even with a tripod you will see wobble in the images. This is why 7X50 or some other 7X binocular is usually recommended. A 7x50 with a big exit pupil of 7 or more has always been the most recommended marine binocular. (Find out more about exit pupil measurements below.)

View my kids in the soccer game or out in the yard: 8x40, 8x42 is probably fine. You don't care if you enlarge every hair on their head. You just need a boost in enlargement so you can view your kids or the players out there in general. But, you do want to see a wide Field of View, no doubt. More about FOV later on.

The 8X part of the the mearurement,you will remember, tells you enlargement or magnification and will make things 8 times bigger. Additionally, if the lighting is good, you don't need the bigger lenses so even X21 and up are on the small size but will probably work fine. If you are watching play by play and really into the kids competitive sports, and will be looking for longer than a few glances, you can go for bigger lenses such as the X42.

Stadium sports: 8x40, 8x42. Same as above. Zoom might be nice though, if you want to zoom in on someone in the stands, so something like 8-10X40 or 8-10x42 would be useful in this case. Remember the zoom spec is the range of numbers on the left side of the X.

Hiking: compacts such as an 8x25, 10x25. You don't get a very big lens with compacts, but they are not a lot of weight to carry around either. So, that's your trade off when choosing binoculars for hiking.

Surveillance and Home Security: Check out binoculars that have a lens on the larger size, which is X40 or larger in their rating. Here's why...

Many of our customers say they want to be able to check out what's going on out in the street in front of their homes. For example, the last time I used my binoculars it was to look outside from my bedroom window and find out what was going on inside a car that was parked out in the street in front of my house at 2 a.m. The street is about 50 yards from my bedroom window. How far is the street from your window? 25 or 50 yards?

Also, for surveillance consider your typical lighting conditions. I usually only want to check things out with binoculars when it's dark or dusk out. In this case, I need a large lens that will capture more light. Also, I might want Night Vision binoculars.

Since X40 and higher designates a larger lens size, that's what to get in this case.

For surveillance around your property, buy binoculars that end in X40 or larger. That's on the large end of lens sizes/diameters for binoculars. For example, 8X40 or some other high number like X50 . If you can get the higher number, the better.

Additionally, zoom binoculars would be nice but it's not required in this case.

How to remember this: For surveillance and low light, get a high number after the X because that is the lens size. 8X50 is better than 8X40 in this case.

Hunting: 8x40, 8x42 or if you have a steady hold, can tolperate wobble or have a tripod 10x40, 10x42 and roof prism. (You can read more about roof prism binoculars below.) Is it damp out on the trails? Does it rain? If you're not in the desert, you want your binoculars to be waterproof or at least fogproof.

How to remember this: When hunting, if you are looking for game or varmints far off like 500-1,000 yards out there, get a high number before the X: 10X40 is better than 8X4. But, if you are in the woods or looking across a field, just to see what's up (and your rifle scope will do the rest) you can use a lower magnification, so 8X may do it for your.

Also, go light, because you have to carry everything! That means you may want a big lens but don't want to carry a humongous set of binoculars. So, get around X20 to X40 for daytime.

But, if you're hunting at dusk, you will need up around X50 or greater and will have to deal with the added size and weight.

With tripod, you can go for the higher magnifications like 12X through 16X. A zoom like 10-22X50 would be fine.

Details About More Binocular Specifications

Want more details? You've come to the right place!

TABLE OF CONTENTS:

REVIEW OF BINOCULAR SPEC NUMBERS

  • Q. How do binocular specs work?
  • A. The typical binocular is labeled with two numbers, for example 8x40, or 7x35 etc. A few binoculars use three numbers as in 10-22x50. These are zoom binoculars.
  • Q. What does the first number in a binocular mean, such as the 8 in 8x40?
  • A. The first number - the 8 - is the magnification. That's how many times the image is enlarged over normal when you look through the binocular. 8 means the image will look 8 times larger or 8 times closer than it really is. An 8x50, 8x40, 8x20 all have a magnification of 8.

    What about something like 8-24X25? Since the number on the left is the magnification, the binoculars go from 8 to 10. These are zoom binoculars.You can change the magnification from 8x up to 24x.

    Here are some interesting things about magnification. As the magnification goes up, the image brightness goes down, field of view (the amount of territory seen through the binocular) goes down and, just as importantly, image steadiness also goes down. More about all these other specs later. However, it's important to know that bigger numbers are not always better due to the trade offs inherent in optics. For example, take wobbliness, which occurs when the spec is 10X or higher. The image wobbles because a binocular magnifies not only the image, but also your muscle movements and you might even see the image jump each time your heart beats. That is why, most people find that a 10X binocular is about as high as they can go without a support or tripod.

  • Q. What does the second number in a binocular mean, such as the 40 in 8x40?
  • A. The second number tells how big the lens is. It measures the diameter (width) of each front lens in millimeters. This number directly affects brightness and sharpness. An 8x40, then, will produce a brighter and sharper image than an 8x25, even though both enlarge the image an identical eight times. The larger front lenses in the 8x40 also let in wider beams of light as it enters the eyepieces (see exit pupil below for more on this topic). This makes it more comfortable to view with an 8x40 than an 8x25.

    On the other hand, the larger front lenses in the 8x40 binocular also make the 8x40 bigger and heavier than the 8x25 binocular. An 8x25 may not be optically as good as an 8x40, but its smaller size can make it easier to carry or store. You may want to make sure your binoculars are not too heavy or big. Or, size may not be a consideration you are worried about.

    The technical term for this number is the Objective. Why call it that? Because it refers to the front lens and, that is the lens that forms the image of the object. So, it's about the object or objective.

FIELD OF VIEW

  • Q. What about Field of View?
  • A. Field of View (FOV) is the amount of territory you see when you look through the binocular. Imagine a fence 1000 yards away. If a binocular has a Field of View of 372 feet at 1000 yards, you will see 372 feet of the fence. If the Field of View is 250 feet at 1000 yards, you will 250 feet of fence.

    Field of View depends on magnification and also the binocular's eyepiece. Generally, when you get more magnification, you will get less Field of View. This means that 10x will show more details on the fence at 1000 yards than 8x will show, but 10x will not show you as wide a section of fence.

    What about eyepiece design and Field of View? Wide-angle eyepieces will increase Field of View, if they are of good optical quality. However, they are pricey and can also cause the image to be less sharp.

    In short, Field of View isn't one of the binocular specification numbers as in 8X42. But, you can get more Field of View if you get a bigger magnification or a wide angle eyepiece.

OTHER BINOCULAR SPECS: EXIT PUPIL, RELATIVE BRIGHTNESS, AND TWILIGHT FACTOR

  • Exit Pupil in a binocular is how big a beam of light comes in through the eyepieces. This is measured in millimeters. You can see the exit pupil by holding the binocular out at arms length, so that you can see a circle of light in each eyepiece.

    If you want to calculate the size of the exit pupil, divide the first binocular number into the second binocular number. A 10x50, 7x35, and 8x40 all have an exit pupil equal to 5 millimeters.

    Exit pupil generally tells you what the image brightness will be. Binoculars with larger exit pupils give brighter images in dimmer light. For normal daylight viewing, an exit pupil of 2.5 or 3 is fine. For low light surveillance, stargazing and astronomy, an exit pupil of 5-7mm is preferred.

    The biggest exit pupil that your eye can handle is actually 7. So, your eye can't really use anything larger, but keep reading for other interesting aspects of what your eye can do with a bigger exit pupil.

    Also, the exit pupil measurement doesn't take into account the effects of lens coatings and optical quality, so it doesn't give you the whole story regarding how good an image you can see with a particular set of binoculars.

    But, consider this! A larger exit pupil is easier to keep centered in your eye, so this ishelpful when it is difficult to hold a binocular steady, as on the deck of a moving boat! That's why a 7x50 with an exit pupil of 7+ has always been the typical recommended marine binocular.

  • Relative brightness (RE) is another general guide to image brightness, because it is really just the exit pupil squared. This means that binoculars with an exit pupil of 5mm will have an RE of 25. It's just another way of measuring the exit pupil.
  • Q. How about Twilight Factor?
  • A. If you are going to be using your binoculars a lot at dusk or in low light, such as dark cloudy days, the Twilight Factor will tell you how much detail you will mostl likely be able to see. It's the square root of magnification times the objective. Therefore, a 10x40 has a Twilight Factor of 20.

    Remember that these measurements: exit pupil, relative brightness and twilight factor are really rough measurements and they do not tell you about quality that can be due to special lens coatings, type of glass and other manufacturing techniquest that make a premium grade binocular superior to lower grade models.

PRISMS

  • Q. Some binocular specs include prism. What is it?
  • A. Without a prism, a binocular would produce an upside down image. Binoculars have either porro prism or roof prism.

    Porro prism is one of the most efficient, least expensive and earliest types of of prism developed for optical instruments. However, a porro prism is larger and bulkier prism than a roof prism, which was developed later on. Because of the weight, the body of a porro prism binocular will also tend to be more bulky.

    You can always recognize a porro prism binocular by its shape. In this case the eyepiece and the front lens are always offset and are never in a straight line. Optically, however, a porro prism is very efficient.

    Since is less expensive to make a porro prism than a good roof prism, a good quality porro prism binocular is every bit as good optically as a quality roof prism at a much lower cost. If you want to get the most optics for your dollar, especially in a binocular under $200, a porro prism is your best option.

    Roof prism is smaller and it's shaped like a little hut or roof. Its main advantage of is its size and shape. Since roof prism is smaller and more compact than porro prism, it can be installed in a much more compact housing. This makes the binocular easier to handle for most folks.

    You can tell a roof prism binocular by its shape. The eyepiece and the front lens are always in a straight line or straight tube.

    Roof prisms are more expensive to make to the same optical standards as good porro prisms.

    Another interesting factoid: Not all roof prisms are made the same or of the same quality. Some are phase corrected and these are the sharpest. Others may have silver coated mirrors, and these are brightest. And, some are aligned better than others.

    A good roof prism binocular is actually more rugged and smoother to focus than an average porro prism binocular, because the roof prism is contained in its own cage and its focus mechanism is usually inside the binocular instead of on the outside.

  • Q. What about optical glass grades like Bak4 or BK7?
  • A. All quality binoculars use Bak4. Only inexpensive binoculars use the lower quality BK7 grade of glass.

LENS COATINGS

  • Lens coatings are chemical coatings on the lenses in a binocular to make images brighter. Whenever light goes through a lens, a little bit of light is lost. Some light gets reflected and some of it gets absorbed by the lens. To minimize loss due to reflected light, special lens coatings are applied. There are three types of lens coatings: (1) Fully Coated Lenses, (2) Multi-Coated Lenses, and (3) Fully Multi-Coated Lenses.

    Fully Coated lenses coated were the earliest lens coatings and these are now the least efficient. The coating is a single layer of magnesium fluoride, and it is found only on inexpensive binoculars. These lenses will not produce images as bright as multi-coated binoculars.

    Multi-coated lenses in handheld binoculars have severak layers of special chemicals. The multi-layered coating allows the maximum amount of light to pass through the lens. A multi-coated binocular may have some lenses coated and others not, but it will always be brighter than a "fully coated" binocular.

    A Fully Multi-coated binocular produces the brightest image of an ycoating system. All quality binoculars are fully multi-coated.

FOCUSING

  • Center Focus Binoculars use a single wheel to focus on objects. It can focus on objects both very close and far away, making it the most versatile and commonly used focusing system in a binocular.
  • Iindividual Eyepiece Focus (IF) Binoculars require you to focus each eyepiece when looking at an object, but once focused for your eyes, objects from 40 yards away to infinity are always in focus and require no additional focusing. This is a great system for medium range and long range objects, but it is not well suited for close in work. IF binoculars are most commonly found in marine binoculars and astronomy binoculars.
  • Focus Free is an economy version of an IE focus binocular, but the eyepieces are locked and set at the factory and cannot be adjusted. This means that you can never focus on objects closer than forty yards away and it also means that the binocular cannot be adjusted for differences in strength between your right eye and left eye. This is a serious shortcoming for most people, since most have one eye a bit stronger than the other.

DIOPTER ADJUSTMENT

  • The diopter adjustment on a binocular allows you to compensate for differences in strength between you right and left eye. Since most people have one eye stronger than the other, this is a feature found on all binoculars except for "focus free" models.
  • Q. Where is the diopter adjustment on a binocular located?
  • A. The diopter adjustment has been traditionally located on the right eyepiece, but many models today use a separate wheel or a locking mechanism on the center focus knob.

INTERPUPILLARY DISTANCE

  • Interpupillary Distance is the distance between the pupil of the eyes. All binoculars can be opened wider or closed tighter to accommodate different widths, though people with very small faces or people with very large faces may still have trouble finding a binocular that will fit them.

CLOSE FOCUS / MINIMUM FOCUS

  • Minimum Focus or Close Focus is the nearest distance at which a binocular will focus on an object. A binocular will not focus on an object that is any closer. This feature is important for some uses, such as birding.

EYE RELIEF

  • Eye Relief is the maximum distance your eye can be from the eyepiece and still see the entire field of view. If your eye is farther back than this distance, you will see a reduced field of view. This is a concern if you wear eyeblasses when looking through a binocular, because eyeglasses alwyas prevent your eyes from getting right up close to the eyepieces.
    The minimum recommended Eye Relief is generally 14 or 15mm. However, if you have eyeglasses with thick glass lenses, you should get 17-20 mm.
  • Q. Should the eyecups on a binocular be in the "up" position or "down" position when using eyeglasses with a binocular?
  • A. Put the eyecups in the "down" position when wearing eyeglasses and in the "up" position when not wearing eyeglasses. (Older style binoculars have rubber fold down eyecups, but most modern binoculars "twist up" or "pull up" style eyecups.)

WEIGHT

  • Q. How much weight in a binocular is too much to carry aournd the neck?
  • A. Usually, over 35 ounces is too much to comfortably carry around the neck. A weight less than 30 ounces is much better. If your current binocular seems too heavy to carry comfortably, you might want to get a binocular harness that is designed to support the weight on your shoulders instead of your neck.

WATERPROOFING AND FOGPROOFING

  • A fogproof binocular is one that is guaranteed by the manufacturer not to fog up due to moisture inside the binocular (any binocular can fog up on the outside). A binocular is made fogproof by filling it with nitrogen. Other labels such as water-resistant, climate-proof, rain-proof are not a guarantee of waterproofing.
  • If you are using a binocular around water or will be using it under harsh conditions, waterproofing is a must. Even for more casual use, though, a waterproof binocular is a better sealed binocular and less likely to develop problems with dirt and dust entering the binocular.

ARMORING

  • An armored binocular is a binocular with a body that is covered by rubber or another synthetic material. Armoring protects it from scratches, makes it more comfortable to hold and also "quiets" the binocular when it accidentally rattles against something (so you don't scare the animals away). Armoring does not make a binocular waterproof.

TRIPOD MOUNTING

  • You can mount a binocular to a tripod if it is listed as tripod adaptable or if it is threaded for a tripod adapter. If it is listed as tripod adaptable, you may still need to purchase a tripod adapter, though a few large binoculars may have this accessory built in.
  • Q. Where is the tripod socket located on a tripod adaptable binocular?
  • A. It is usually located at the front of the center hinge where the binoculars fold. It is often hidden under a cap.
  • Q. When should I attach a binocular to a tripod?
  • A. Any time you have a binocular magnification over 10x or 12x, you should attach a binocular to a tripod to steady the image. Also, heavy binoculars with a last number of 70 mm or more usually need a tripod to support the weight.

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