Catoptrics Resources and How-To Tips: Catoptrics (formerly anacamptics) is the branch of optics which explains the properties of incident and reflected light, and particularly that which is reflected from mirrors or polished surfaces.

Binoculars, Riflescopes, Spotting Scopes -- Optical Instruments & Accessories


Recommended Books

Using Binoculars for Astronomy

Binocular FAQ

What does "Catoptrics" mean in our title?

Why own binoculars? For a host of reasons. Here are a few:

• Binoculars are great for watching sports. A Zhumell Zoom binocular can put you right in the action.

• Binoculars are essential for birding. A pair of 10X Zeiss 10x42 FL-T binoculars can get you so close you can almost count the feathers.

• Binoculars are great for hunting. Nikon Monarch binoculars will allow you to separate branches from antlers and give you a clear shot at a trophy.

• Binoculars also are great for mariners. Scope out snags with Bushnell Marine binoculars.

Binouculars for Bird-Watching

The most popular binocular sizes for bird watching are 8x40 and 8x42. Binoculars with these specs are compact enough to carry with you all day while large enough to gather plenty of light. Other features to look for in birding binoculars are a close focus for near observation of birds and insects, a wide field of view, enabling you to track quick-flying birds and to see more individuals at a time when counting or observing large flocks or groups, such as kettles of hawks, and good depth of field so you don't need to keep adjusting the focus to keep a small bird in focus, and don't need to turn the focus adjustment too much to go from near to far. top of page

Binoculars for Hunting

As with the marine binocular, the hunter’s model should also be waterproof as well as rugged. Higher power is recommended because distance is often a key in hunting and you need to be able to see your target clearly. 8x42 and 10x42 are common choices, with some sort of more rugged housing to take potential abuse in the field.

Binoculars for Marine Use

For boating or any marine use, waterproofing is critical. The housing should be solidly built enough that it can take some punishment when it gets really rough. A power of 7 with a 50mm objective lens is most common for a marine binocular. This allows for a large field-of-view and an ample objective to collect a lot of light. Another feature of many marine type binoculars is a built in compass, perfect for navigation.

Zoom Binoculars

For ultimate utility, zoom binoculars are what you want. Zoom binoculars offer users a wide field of view and bright image at low powers and zoom in and view distant details quickly and easily. The key to the zoom type binoculars is the ability to change the power. In a 9-27x50, you can focus in a 9x50 and zoom it all the way up to 27x50 with the flip of a lever. This is great facility to have when you want to be just a little close to the subject.

Using Binoculars for Astronomy

Not interested in the complexities of a telescope? A good pair of binoculars can bring the heavens closer in a much easier to use package.

Binoculars give you the advantage of using both eyes for a more three dimensional stereo view. Binoculars can be very good for observing the moon and stars. The Orion Nebula and Andromeda Galaxy are easy to spot on a dark clear night. Even Jupiter and its Moons are visible through a pair of binoculars. top of page

astronomy-suitable binoculars

Choosing a binocular for Astronomy

When choosing a binocular for astronomy you first need to understand how binoculars work. Similar to telescopes, a binocular needs to gather light. The same critical feature for telescopes is the same in binoculars - you need a large enough objective lens to gather light. top of page

Binoculars are measured with two key features - its magnification and its objective lens. For example, a binocular may be listed as 10x50. The first number 10 is the magnification of the binocular. The second is the size of the objective or outside lens in millimeters. 10 times the naked eye with a 50mm objective. top of page

Like telescopes, objective is the most important factor. It's the same for binoculars for astronomy. We recommend a minimum of a 50mm objective lens for astronomy. A 7x50 or 10x50 are very common choices for astronomy. They offer a large enough objective lens and a magnification that is enough to bring objects close enough to observe. top of page

Giant Binoculars

Even better for astronomy is the larger objective lens binoculars. Many astronomy binoculars will features objective lenses between 60mm to 100mm or even more. These larger binoculars will usually require a tripod, as they are very heavy. The larger objective size binoculars will often have much more magnification than traditional sized binoculars. Powers of 10x, 15x, 20x or more are common on larger astronomy binoculars. When you have binoculars of this size and magnification - having a steady hand is usually not enough. Having them mounted on a tripod will give you the best results. Many giant binoculars will have a built-in tripod mount or have an adapter included or sold separately. top of page

Image Stabilized Binoculars

image-stable binoculars from Canon

One of the greatest advancements in binoculars is the Image-Stabilized binocular. Brands such as Canon and Nikon have developed a revolutionary method of stabilizing the image with a tiny microprocessor inside to counter the movement of your hands. These binoculars are amazing for astronomy as you can have all the advantages of high power magnification without the need of a tripod. top of page


All tripods have three things in common: legs. From there they diverge markedly. You'll find aluminum tripods, carbon-fiber tripods and tripods made of steel. There are tripods for binoculars, for cameras and for telescopes. And we have them all. Check out our large selection; your optics will thank you.

"Catoptrics" defined:

Catoptrics (formerly anacamptics) is the branch of optics which explains the properties of incident and reflected light, and particularly that which is reflected from mirrors or polished surfaces. Binoculars and reflecting telescopes are catoptric instruments.

Catoptrics was also the title of the first known book on the science of optics, authored by Euclid (300 B.C.E.). top of page

Recommended Books

The Backyard Astronomer's Guide

The Backyard Astronomer's Guide
by Terence Dickinson, Alan Dyer

Despite the book's title, there is very little about astronomy here, i.e., lists of constellations, star charts, night sky maps, or details about planets, stars, and galaxies. However, there is a wealth of information about the equipment used in astronomy, including prices, consumer-type information, advice on when to use and when not to use binoculars, telescopes, cameras, film, lenses, filters, and other items for the amateur astronomer. Four chapters, though, concern the observation of the solar system and deep space objects. There are also several chapters discussing the photographing of all types of astronomical phenomena. Though cost may deter small-to-medium-sized libraries, there is much information here for the experienced amateur, and some useful information for the beginner as well. (Illustrations and index not seen.) -- Robert Ellis Potter, Dunedin P.L., Fla.
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Book Description:
What type of telescope is best for beginners? Can I use my camera to take photographs through a telescope? How good are the new computerized telescope mounts? What charts, books, software and other references do I need? These questions are asked time and again by enthusiastic new amateurs as they take up recreational astronomy. top of page

But accurate, objective and up-to-date information can be hard to find. Throughout the 1990s, the first edition of The Backyard Astronomer's Guide established itself as the indispensable reference to the equipment and techniques used by the modern recreational stargazer. Now, authors Terence Dickinson and Alan Dyer have produced an expanded and completely updated edition that again sets the standard for accessible and reliable information on one of the world's most popular hobbies. top of page

Dickinson and Dyer -- both full-time astronomy writers -- bring decades of experience to their task. They explain why telescopes often perform much differently from what the novice expects. They recommend the accessories that will enhance the observing experience and advise what not to buy until you become more familiar with your equipment. They name brands and sources and compare value so that you can be armed with the latest practical information when deciding on your next purchase. Sections on astrophotography, daytime and twilight observing, binocular observing and planetary and deep-sky observing round out this comprehensive guide to personal exploration of the universe. Dickinson and Dyer's elegant yet straightforward approach to a complex subject makes this book an invaluable resource for astronomers throughout North America. top of page

With more than 500 color photographs and illustrations, The Backyard Astronomer's Guide is also one of the most beautiful -- and user-friendly -- astronomy books ever produced. top of page

Amazon reviewers say:

"This classic introduction to amateur astronomy has gotten a much improved revised edition. Dickinson and Dyer have updated this indispensable resource for the 21st century. This book is divided into three main parts.

The first covers the hardware. The authors explain the workings of the different types of scopes and accessories and give suggestions based upon budget and the type of viewing to be pursued. They tell you what hardware is essential, what is nice to have, and what you can live without including new scopes and accessories that have come out since the previous edition. Also covered is how to set up the scopes properly, and what mistakes to avoid setting up a new scope for the first time.

The second part is a crash course on the sky, starting with what you can see with the naked eye, observing conditions based on your location, and then how to observe the various objects in the sky with the equipment from the first part.

The final part is an introduction to photographing the sky with a camera (film or digital) or a dedicated CCD imager. Coverage includes simple camera on a tripod or barndoor mount setups, piggybacking on a telescope, and thru the telescope photography. Enough to get one started.

The text is not the only part of this book to be updated. Hundreds of color photos have been sprinkled liberally throughout this guide. If you are just getting one book before plunging into amateur astronomy, make this the book. It is great for beginning and intermediate amateurs."

Test Optics Before You Buy!

“.. .After a scope takes a hundred hits, it’s good for thousands.”

RIFLESCOPES, BINOCULARS AND spotting scopes are your windows to what’s a field. Buy good ones. As a rule, the more you pay, the better the glass. But to make sure your money goes as far as it can, put those optics through simple tests before you write the check.

1. Visit a store that has a broad selection of optics, to compare same-price products under the same conditions. Then take the optics out of the store. You need to know how they per form in natural light, at a distance. Insist on fully multi-coated lenses. You can’t tell by looking if all the lenses are coated or if any is multi-coated; that should be specified in product literature. Coating makes the image brighter. Up to 4 percent of incident light is lost at every uncoated air-to-glass surface. Time your test so you can look into deep shadow and obliquely toward low sun (don’t look at the sun!) You want a bright, sharp image at dusk and in timber, and when game is backlit.

2. Using a signpost or power pole as a target, move the binocular or scope side to side, watching the field edge. The target will curve slightly as it reaches the edge, but excessive distortion is a liability. Very flat fields can be achieved with a “stop,” but at the expense of brightness.

3. With the instrument held stationary, move your eye forward and back behind the ocular lens. You want comfortable eye relief (the distance between your pupil and the lens when you see the full field). You also want latitude in the eye relief. A scope or binocular with critical eye relief gives you only a partial field unless your eye is an exact distance from the lens. Check the eye relief of variable scopes throughout their power range. On many models, eye relief becomes more critical (and shorter) at high magnification. Scopes with less than 3 inches of eye relief can give you recoil scars.

4. Test for parallax in a scope by fixing the reticle on a small object far away and, with the scope still, moving your eye side to side. Now move your eye up and down. As the field blacks out, you may see the reticle move off-target. At one specific distance (generally 100 or 150 yards, closer for scopes designed for rimfire rifles), the factory has set the scope to be parallax-free. At other distances, some parallax error occurs. Scopes with adjustable objectives enable you to correct for parallax. Parallax can cause you to miss because the bullet goes not where you see the reticle, but where it was when your eye was on the scope’s axis. But don’t make too much of parallax. If your eye is centered behind the scope, it has no bearing on your shot.

5. Any spotting scope with fluorite glass — the “ED” or extra-low dispersion model — costs a lot more than the standard version. Test the resolution of both on a distant, dimly lit object. The difference may not be worth the premium. Because high magnification ensures a small exit pupil (and limits brightness at dusk), you’ll want a big front lens on a spotting scope. Plus a sturdy, quick-adjust tripod.

6. Move all movable parts to check for looseness and binding. Scope adjustment clicks should be crisp and uniform. Focus wheels and diopter rings on binoculars must spin smoothly but not too easily. Turning the ocular housing on riflescopes does not adjust target focus; it only sharpens the reticle. Point the scope at the sky, and rotate the housing until the reticle comes clear. (On traditional American scopes, you’ll need to loosen the lock ring and turn the whole ocular housing; not so with European-style helical sleeves.) Now close your eyes, open them and refine. Looking at the reticle too long or aiming the scope at a target, you force your eyes to focus. You want a sharp reticle when your eyes are relaxed.

7. Finally, read the warranty. You can’t test the durability of an instrument before you buy. The warranty tells you if the manufacturer built it tough.

Sporting Optics Guide

Optics for the Outdoors

Few people who use optics have the background — or interest — to explore how light has been harnessed by glass. Most of us are just delighted to see better. Sight is, after all, our primary sense. Unlike the deer we watch, we have only a rudimentary sense of smell. We can’t detect subtle changes in temperature like a snake or echolocate like a bat. What we don’t see goes largely unnoticed. Almost miraculously, optical glass gives us images that seem brighter, sharper and bigger than life. We who study wildlife and hunt game benefit most. Binoculars let us see details we’d otherwise miss — details that tell us volumes more about our surroundings than we’d otherwise know.

Scopes and red-dot sights help us hit targets that would be obscured by iron sights, indistinguishable or just plain invisible. With scopes, old eyes can compete with young eyes on the hunt and in shooting competition. Spotting scopes help us identify and assess the trophy quality of distant game animals and pick the best approach through difficult places. At the range, they show us our bullet holes, as well as the mirage that explains why they aren’t all in the middle.

This web-site will tell you as much about the selection and use of outdoor optics as any that I’m aware of and more than most. After 35 years of hunting, competitive shooting and wildlife study, I’m still learning about optics. But because knowledgeable people have told me a lot about binoculars, riflescopes and spotting scopes, there’s plenty of information here. Some of it has come from obscure scientific texts on light and optical glass. It’s been my good fortune to use scores of new and old instruments a field. You should find in the many anecdotes some that remind you of your own hunting and shooting experiences.

Look here for practical help on evaluating optics — beginning with an explanation of industry lingo so you understand the terms used to describe the products. There’s no-nonsense talk about price: How can a scope that costs five times as much as another scope be five times as good? And if it isn’t, how can the expensive model sell? Do you really get better performance from binoculars with a blue-blood European pedigree? And what makes ED spotting scopes cost so much? You’ll learn why it’s hardly ever necessary to pay retail price for new optics and what to keep in mind when shopping for used glass.

The best instrument delivers its potential only when you learn to use it expertly. Here you’ll find tips from optical engineers, big game guides and competitive marksmen — people whose busi ness demands that they get the most from lenses. You’ll learn how to test binoculars, scopes and spotting scopes before buying, how to adjust them in the field and how to ensure that they give you top service for many years.

There’s a catalog section in this web-site too. Not a raw compilation of all products available, but a carefully sifted list of optics that excel or are especially good bargains. Who picked ‘em? Me. The idea was to offer photos, descriptions and prices of items you might find useful and, by virtue of price and performance, particularly attractive. I did my best.

This web-site was several years in the making. I hope you find its text informative, the anecdotes entertaining and the illustrations helpful. And that your next trip a field is the best that good glass can make it!

How to Choose Binoculars Most Suitable for Your Needs

This article has been written to help you learn how to evaluate binoculars and then show you how to choose binoculars based on your budget and the purpose for which you plan to use them.

We shall be looking at how binoculars work. Understanding this will help you to understand the different factors that will affect price and the features that are available - usually these revolve around optical quality, magnification power, and portability – before moving on to look at different price categories and justifications for buying a more expensive pair depending on what they will be used for.

The overall aim is to prepare you for the task of finding the right pair of binoculars, at the right price, without compromising on features that you may require. top of page

How Binoculars Work
Essentially, all binoculars and scopes are derived from classical telescopes, which consist, in their most basic fashion, of two lenses. The lens nearest whatever is under scrutiny (objective lens) provides an image, which can then be enlarged by the lens nearest the viewers eye (eyepiece lens), by moving it closer or further away from the objective lens. top of page

A pair of binoculars can be seen as two such telescopes, side by side, which together produce an image which has the depth of field that we are used to, rather than just a large flat image. top of page

Since the light has been refracted (bent) as it has been directed through the lenses, by the time the viewer sees the image it is back to front, and upside-down. To correct this, two prisms are placed inside the binoculars, between the objective and the eyepiece. It is the presence of these four prisms in the shoulders of the binoculars that give them their squat appearance. top of page

Power, Light and Weight
The power of the optics is expressed as two numbers, such as 7 x 35. The first is the number of times magnification, and the second is the diameter of the objective lens. A larger objective lens makes sense during low light conditions, since it can capture more of the available light. top of page

The magnification factor tells you how many times larger the object will be magnified – a number of between 4 and 7 is ample for most applications. Any larger than about 9 or 10, and the natural shake of the human hand will be magnified to such an extent that the image becomes difficult to see, and a tripod will be required. top of page

Glass also has a tendency to reflect as much as 5% of the light that arrives at its surface back towards the light source. A simple coating was devised to prevent this, by allowing more light to pass through the lens, and less to be reflected back. Since the advent of the original coating, the technique has been refined, and there are several grades of lens coating available. top of page

The best result is achieved when multiple layers of coatings are applied, to the front and rear of the lens. Each coating is designed to provide the maximum transmission of light through the lens, and minimum reflection and diffraction, resulting in a brighter, clearer picture than with standard non-coated lens models. top of page

Modern lightweight binoculars have also evolved in terms of the use of roof prisms, rather than the traditional Porro prisms. This means that they have no ‘shoulders’ and look more modern. The lack of superfluous casing makes them easy to carry, and substantially lighter than traditional binoculars, however the price tag for higher power models tends also to be more substantial than for the traditional type of a similar magnification. top of page

Pricing Justification
When considering how to choose binoculars price is a major consideration. There are several factors that will affect the price. The first is the type of lens and coating that is used; glass lenses, which are coated on each side with multiple layers, will produce a picture at high magnification which is substantially clearer and brighter than that produced by plastic lenses. top of page

Plastic lenses, on the other hand, tend to make the binoculars lighter, but will be substantially more expensive for the same grade of picture quality. If the binoculars are to be used in clear conditions, at a low power, then this may be acceptable. If more variation in lighting (i.e. dusk and night use) is expected, then one should opt for better quality optics, and hence a higher price tag. top of page

The build quality will also affect the price. More rugged, shock-proof binoculars destined for use in harsher conditions (marine or backpacking) will cost more than those which do not need to be waterproof or shockproof. top of page

Uses and Solutions
Before you decide how to choose binoculars you need to consider the solutions for differing environments. Single scopes or spotting scopes, for example, are often used for hunting and bird watching. Here, since they are, in effect, half the size of a regular pair of field glasses, better quality optics can be afforded, as the cost will be proportionally lower. top of page

Bird watching glasses need to be good in all light conditions, from dawn to dusk, and even have limited night vision. The same goes for hunting glasses, and in both cases they should be lightweight, but probably with a smaller magnification, and larger objective. top of page

High power spotting scopes, or binoculars, where the power exceeds 10x will need to be mounted on a tripod. The best models will be ones with a very large objective lens, suitable for use in many conditions, but will be too heavy and cumbersome for use on the move. top of page

Finally, when hiking or orienteering, binoculars will be very useful, but it is important to note that optics are very fragile, and so plastic lenses over glass ones, and a rugged case are probably going to be more important than high power, or the ability to use them at night. top of page

As a curio, it is possible to buy, from Zeiss, a pair of binoculars which have a mechanical anti-jog mechanism which allows for extremely high magnification, but without the shake associated with it. They come in at around $4000.

Digital binoculars are a cheap alternative, and can be picked up for considerably less ($200), and usually have a built-in camera. They are not perfect optically, with a resolution of around 3 megapixels, but will suffice for the hobbyist. top of page

Jargon Explained
There are many different terms that are bandied about when reading descriptions of binoculars and before rushing off to the store, it is worth understanding some of the more esoteric ones. top of page

For example, there are several different descriptions of the coating that has been used (see Power, Light and Weight) to reduce the amount of light reflected back through the lenses during magnification: top of page

· C : Some surfaces coated
· FC : All surfaces coated, except plastic lenses
· MC : Some surfaces have been coated in multiple layers
· FMC : All glass surfaces are coated with multiple layers.

In the last case, one would expect a good quality piece of optics to be able to transmit between 92% and 95% of all available light back to the eye. top of page

The “Exit Pupil” is also important and can be calculated by dividing the power by the objective size and yields a value which is very important – it is the diameter of the light fed to the eye. Given that the average human pupil ranges in size from 2mm to 7mm depending on the available quantity of light, it is clear that, in the midday sun, an Exit Pupil value of 4mm (for example) will mean that 50% of the image returned to the eye is lost. top of page

By a similar token, if the value is smaller than 7mm for a night scope, then it is not taking advantage of the human anatomy. A word of advice – always use night scopes in the dark, to keep the pupil as wide as possible; this means no, or very low, light around the place that you choose to hide out whilst communing with nature. top of page

Finally, if the phrase ‘Eye Relief’ is mentioned, then it refers to the way that the eyepiece is set up with respect to the other optics in the device. Most glasses will come with eye relief between 9mm and 13mm, and is the distance from your eye to the lens before your field of view becomes limited. top of page

If you wear eyeglasses, then eye relief above 14mm becomes desirable, since you will already have a certain amount of distance between your eye and your eyeglass lens, which you can not change. Given this, if your chosen optical device has a small eye relief, then you will have a very restricted field of view, and miss out on most of the picture! top of page

As you can see when you need to decide how to choose binoculars most suitable for you there are many aspects to consider. Hopefully this article will have made your job easier. For more information on the binoculars available you can visit the relevant links. top of page

Binoculars: Frequently Asked Questions

1.   What is magnification?
2.   What is the Objective Lens?
3.   How do you focus your binoculars?
4.   What is the field of view?
5.   What is the close focus?
6.   What is the exit pupil?
7.   What is relative brightness?
8.   What is eye relief?

Q: What is magnification?

A: Magnification:

Magnification is the degree to which the object is enlarged. With a 7x35 binocular, the object will appear to be seven times closer than without the aid of a binocular. The number immediately following the "x" is the diameter in millimeters of the objective (front) lens. The larger the front lenses the more light that is allowed to enter the binocular. A 10x50 binocular has 10 times magnification with a 50mm objective lens.

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Q: What is the Objective Lens?

A: Objective Lens:
Objective Lens

With a 7x42 The number immediately following the "x" is the diameter in millimeters of the objective (front) lens. The larger the front lenses the more light that is allowed to enter the binocular. A 10x50 binocular has 10 times magnification with a 50mm objective lens. This photo is showing an objective lens that is 42 millimeters.

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Q: How do you focus you binoculars?

A: Individual eye strengths vary. Please refer to the instructions below for your individual type of binocular. CENTER FOCUS and INSTA-FOCUS® 1. Adjust the interpupillary distance. 2. Set the diopter setting (normally on the right lens) to zero and view a distant object. 3. Keep both eyes open at all times. 4. Using a lens cover or your hand, cover the objective (front) lens of the side with the diopter setting ring. 5. Using the focus adjustment, focus on the distant object being viewed. 6. Cover the other objective lens and view the same object as above. 7. Using the diopter setting adjustment ring, focus on the same distant object. 8. Your binocular should now be adjusted for your eyes. Make a note of the diopter setting for future use. - Notes: Zoom Binoculars should be focused at the highest power possible. Perma-Focus Binoculars do not require adjustment and use your eye's own ability to accommodate. Most users have no difficulty with these models.

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Q: What is the field of view?

A: field of view

The field of view is the width, measured in degrees or feet, of the viewing area you would see at 1000 yards. Field of view is generally affected by the power of the binocular compared to the objective lens size. A larger power (if the objective lens size stays constant) will lead to a reduced field of vision. A larger objective lens (if the power stays constant) will lead to a larger field of view.

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Q: What is the close focus?

A: The Close focus is the closest distance from the object that the viewer must be before they can focus the binoculars on the image.

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Q: What is exit pupil?

A: exit pupil

The exit pupil refers to the size of the shaft of light transmitted to the eye. The larger the exit pupil, the brighter the image will appear. The exit pupil is an important indicator of the binocular's low light performance. You can actually see the exit pupil by holding the eyepiece of the binocular approximately 12 inches from your eye. It is the bright circle of light in the center of the eyepiece. Exit pupil is expressed in millimeters and is normally derived by dividing the power into the objective lens diameter. A 7x35 binocular has an exit pupil of 5mm (35/7). A 15-power binocular with a 60mm objective lens diameter has an exit pupil of 4mm.

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Q: What is relative brightness?

A: The relative brightness index is used to compare how well binoculars with different size exit pupils will perform under dark conditions. This index reminds us that as the size of the exit pupil increases, its area and ability to transmit light grow geometrically. To find the relative brightness, square the exit pupil. A binocular with an exit pupil of 5mm has a relative brightness of 25 (5x5=25). Because relative brightness does not consider factors such as optical quality or coatings, it should be used only as a rough guide.


Q: What is eye relief?

A: eye relief

The distance between your eye and the eyepiece is called "eye relief." Extended eye relief is one of the three most critical performance factors, along with magnification and field of view, especially for those who wear eyeglasses in the field.

Binoculars for Hunting

As with the marine binocular, the hunter’s model should also be waterproof as well as rugged. Higher power is recommended because distance is often a key in hunting and you need to be able to see your target clearly. 8x42 and 10x42 are common choices, with some sort of more rugged housing to take potential abuse in the field.


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Updated: Monday, 2023-06-12 11:47

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