Binocular Buyers Guide

Article Source: Best Binocular Deals. Copyright 2008
Date: July 29, 2010

Selecting the right pair of binoculars can be challenging, given the range of possible choices. Do you want light-weight but medium magnification optics, or should you sacrifice ease of portability and economy for higher magnification? What does the term "field of view" and the specification 7x50 really mean? This article will simplify the process of selecting the right pair of binoculars for you by answering these questions WITHOUT getting too deeply into the technical aspects.

Buy Your Binoculars To Suit Your Pattern Of Use - Part I

When I was young my father owned a pair of high quality binoculars that he probably purchased for viewing horse races, though I don't recall him ever using them for any particular purpose. They were heavy, with glass optics (the only kind of lens material available back then).

Sometime in the late seventies I began using them for star gazing. I would go out late at night and lay on the ground behind our house in the shadowed area away from the street lights. I lived in New Zealand then, and the sky was crystal clear. You could see the Milky Way with the naked eye, and the use of binoculars laid open the night sky for fantastic viewing. I remember being able to see globular clusters and stare in amazement, knowing that I was looking at fully formed galaxies possibly millions of light years away.

I'm telling you that story because I have fond memories of those pair of binoculars, and I might not have developed the huge interest in science that later occupied my life if my father had simply purchased the cheapest pair of binoculars that he could lay his hands on at the time. When you buy a pair of binoculars, you'll probably own them for the rest of your life, and your kids may well inherit them, if not use them immediately. So get the best ones that you can afford and you won't regret it, no matter what you intend to use them for.

Having said that, what you WILL be using your binoculars for primarily should play a big part in deciding which pair is best suited for you. You'll only be able to make the right decision once you appreciate what the various factors are that make up a particular pair of optics. So, whether you will be using your binoculars for purposes of Birding, Hunting, Mariner, Sporting, Theater, or Astronomical viewing, you will first need to know some basics. We'll cover those in this article, and then at the end of it you can apply what you have learned to selecting the pair of binoculars that is right for you.

Binoculars By The Nunbers = Magnification X Lens Diameter

The first numbers you'll likely come across when shopping for a pair of binoculars relate to the power of the optics. You'll see a pair of number quoted, like 7 X 40.

The first number is the magnification factor, which in this case is 7. This means that objects in the field of view appear 7 times larger than would be seen with the naked eye. For regular binoculars the magnification factor will likely be between 7 and 10. For really powerful astronomical binoculars that number might go above 20.

The second number in the specification is the width of the light-collecting lens, or aperture, in millimeters. The bigger the lens, the more light that can be collected, so the brighter the image. If you will be doing low-light observations, like in the evening, or at night, the bigger the lens, the better the image. Of course, the trade-off is that the weight of a lens increases as the cube of the dimensions.

So if one lens which is 30 millimeters in diameter weighs 200 grams, another lens of the same design but with a 50 millimeter diameter will likely weight about 100 * (50/30)**3 grams, or 462 grams. Clearly that's a big difference: 1/10th of a kilogram versus almost half a kilogram. Obviously the actual weights depend strongly on the cut of the lens and the optical material (glass or plastic) from which it is made. But this demonstrates how the weight of a pair of binoculars can grow as the Lens Diameter number increases.

You'll have to weigh the benefits of brighter images with the cost of lugging around the extra weight. For day trips outdoors, where you simply want to be able to hang the binocular around your neck, you want something lightweight, so probably nothing larger than about 35 millimeters. Also, plastic lenses are a good deal lighter than glass lenses (though a lot more expensive), but we'll get to that later.

If you won't have to be constantly carrying your binoculars, because you'll have a tripod mount for stationary viewing, or you are sitting somewhere (in a stadium, say) and can put the glasses down for a while, then you can afford to go larger in the aperture size.

Another thing to note about the aperture size is that the field of view decreases as the aperature increases. We'll cover what that means in the next section.

The Meaning Of The Term "Field Of View"

When you are using your binoculars the visible area that you can see is limited in extent. This is what is known as your "field of view". In television shows when the director wants to show the field of view seen by some character looking through binoculars it is not uncommon to see two superposed circular areas of viewing with everything outside of them blacked out. In reality it doesn't quite work that way, but you get the picture.

Generally, as you increase the size of the objective lens, or aperture, the field of view narrows. Basically there is less curvature on the surface of larger lenses, so while they capture more light, they angles from which light can be collected are fewer, so the field of view decreases.

Often you will see that the field of view is characterized by a distance, a number like 200 feet. What this means is that if you were to look at a wide object situated 1000 feet in front of you, the distance from the left to the right part of your field of view would be 200 feet. So, if you were in a chopper over the Bering Sea doing rescue operations, a wide field of view would help ensure that you didn't miss somebody lost at sea. But if you are tracking objects in known locations, the actual extent of your field of view matters little to you.

If you are more interested in looking at the night sky, and you read binocular reviews from star gazers, you will see that they almost invariably discuss field of view in terms of the angular range of view, which is typically around 4 to 6 degrees for astronomical binoculars. The larger the optics, the smaller the angular range, and the harder it is to maintain a tremor free grip on your glasses (see the section on image-stabilizing optics below).

So the take away point here is that if you will be scanning for something, like a bird in the sky, a larger field of view is going to be useful. If you will be pointing your binoculars at a known star constellation, you are probably quite happy with a narrow field of view.

The Meaning Of The Term "Exit Pupil"

If you come across the term "exit pupil" it may seem very strange, especially if someone attempts to describe it and does a bad job. Like the aperture size, the exit pupil is just a diameter quoted in millimeters - the diameter of the beam of light that leaves the binoculars and enters your eyes.

Why might the exit pupil be important? Well, in bright sunlight your pupils contract down to about 2 millimeters in diameter, so if the exit pupil is 4 millimeters, much of the image will fall on your iris instead, and never enter your pupil. Likewise, at night, when your pupils open fully, say to 6 millimeters, the image will fall on a smaller part of the light receptive part of your eye than it could have, so the image will appear smaller.

Therefore, if you will be doing most of your observations under more or less exactly the same light conditions, it might be a good idea to get optics with an exit pupil that more closely matches your expected pupil size under those lighting conditions.

Binocular Prisms: Porro verus Roof

Really, you don't need to worry about understanding the difference between "porro" and "roof" prisms when it comes to buying binoculars.

But just for the record, roof prisms are a more modern design that allows the eyepiece to appear directly behind the optical barrels (as shown in the model on the right), so the overall design is more compact. On television you'll see these kinds of bioculars used by military personnel when they are surveilling the enemy on the other side of the gulch! They are popular wherever the use of lightweight binoculars comes in extra handy.

Porro prisms are used to invert the focused image (which is already inverted once by the optics themselves) so that it arrives at your eye looking like the scene in front of you. Porro prisms are the kind that would have been used in my Dad's binoculars from the sixties, but the design is still used today. You'll recognize porro-assisted binocular because the two eyepieces are generally close together, whereas the light-collecting barrels are not. The porro prisms redirect the light inward to the eyepieces.

Image Stabilizing Binoculars

If you regard a pair of binoculars as a serious piece of equipment for your viewing habits, whatever they may be, then you'll definitely want to look into what is available in the way of image stabilized models when it comes to choosing your next pair of binoculars.

As the name suggests, image stabilization is all about keeping the field of view stable during your observations, even if the pair of binoculars themselves cannot be kept perfectly still. Maybe you are in a moving vehicle, or your arms are tired, or you are yourself shaking from the cold. If you are just holding a large pair of high-magnification binoculars with a narrow field of view you are going to notice that hand tremor affects your ability to view with ease.

An image stablized, or vibration reduced, set of optics is going to make your viewing a whole lot more enjoyable. When you consider the increased benefit stretched over the lifetime of the binoculars, it gets a whole lot easier to justify the extra money required to go this route.

So how is it possible to achieve this kind of stabilization? It almost seems like magic if you don't understand the technology, and that's probably fine for most of us, so long as it works well. And it does. The short version of the story is that the way you compensate for a wobbling optical barrel (which makes the objective lens tilt back and forth) is that you allow one or more of the internal optical elements to move in just such a way that the light coming through the lens ends up in the same place, at the back of your eye, as it would have had there been no wobbling of the objective lens.

I'm going to cheat here and show you how this works in an image stabilized lens attached to a digital camera. The principle is the same, and this animated image from Nikon demonstrates the basic magic. Notice how, when the vibration reduction is switched on (wait for the VR letters to turn red), the optical element in the center of the lens moves in just the right way to keep the point of light fixed on the back of the camera. The same thing kind of mechanism keeps light on track in image stabilized binoculars. You switch on the image stabilization when you want it to go to work.

Eye Relief

Eye relief refers to the maximum distance back from the eyepiece that you can position your eyeball and still see the image delivered by the binoculars. The greater the eye relief, the better, particularly if you wear glasses, or the eyepieces have folding covers for protection that you prefer to leave in place.

Typical eye relief may be in the range of 12 ro 15 millimeters, but larger optics can have an eye relief that exceeds 20 millimeters for very comfortable viewing. Astronomy buffs who mount their heavy binoculars favor models which generous eye relief.

Factors That Affect Pricing

Optical Coatings
If you have looked at the marketing images for binoculars you'll have noticed that the more expensive glass-lens models often have what appears to be colored lenses. Sometimes green, sometimes orange, or red. What you are seeing are the optical coatings on the surface of these lenses. The coatings are there to reduce the percentage of light that gets reflected by glass. The less light reflected, the more transmitted, and the brighter the image that falls on the eye. You'll see terms like FC for fully coated lens (single layer), and MFC for multiple full coatings (which means that all the glass elements in the binoculars have multiple coatings - these are the best optically coated glass lens binoculars). The optical quality of such lenses can be much better than plastic lenses.

Optical Material
Glass has traditionally been the material from which the lens elements of a pair of binoculars are created. But lightweight plastic lens are to be preferred if the image quality approaches that of a glass lens of the same magnification. The reason, of course, is the weight difference. If you can reduce the weight of your binoculars by a factor of two you'll be a lot happier at the end of a day spent lugging binoculars across a landscape. But for this added convenience you will pay more. Good high quality plastic lens binoculars are going to set you back more than glass binoculars would because it is difficult to create plastic lenses with real optical quality. Depending on usage, this may be worth it to you.

Magnification
More magnification means larger lens elements, which are a great deal more expensive than small lenses. If you want to view the night sky with binoculars that have 25 times magnification, or higher, expect to pay several thousand dollars for the privilege.

Image Stabilization
Building anti-vibration mechanisms into lenses increases the price of the binoculars significantly. Most image-stablized models are above a thousand dollars, and some models will set you back several thousand. Are these binoculars worth the money? Yes, every cent of it if you can afford it. If you have watched documentaries on television where the video was obviously shot from a helicopter, you may be amazed at how perfectly smooth the image seems to be despite the fact that the helicopter itself is obviously shuddering a great deal. This is achieved with gyroscopic elements that image-stabilize the large filming lenses - exactly the same kind of thing that goes on with an image-stabilized set of binoculars.

Build Quality
Marine binoculars are built to resist the elements. Sea salt air. Water. The knocks and tumbles associated with climbing in and out of boats and navigating shorelines. Because of this they will cost more, but if you treat them right they are going to last a lifetime - and if you don't treat them right, they'll still last a lot longer than your regular pair of binoculars.

Buy Your Binoculars To Suit Your Pattern Of Use - Part II

Now that you have learned the basics of what makes a pair of binoculars suitable (or not) for your own viewing habits, you will be able to see why the following recommendations for bioculars tend to be made for the associated activity:

• Birding/Hunting - 8 X 40 to 10 X 42 - You'll be constantly on the move so you want lightweight optics. Good plastic lenses are optimal here.
• Marine - 7 X 50 - You want optics that are large enough to help you find things potentially quite far away, and sometimes in overcast or low-light situations. Because you may be standing on the deck of a boat bobbing in the water, image-stabilized binoculars are highly prized here.
• Outdoor Sports - 7 X 35 - You usually won't be within 30 meters of the action, so you'll want a reasonable magnification. But most often you'll be watching sports in the middle of the day with plenty of light, so you don't need huge optics. Images will be bright and clear with a medium sized objective lens.
• Theater/Indoor Sports - 6 X 30 - You are always close to the action, maybe 20 or 30 meters from the object you want to see more clearly, so high magnification is unnecessary. You also want optics that are light and small so you can tuck them away easily.
• Astronomy - 7 X 50 and larger - Usually you'll find that when looking at the heavens, the more magnification, the better. So at the high end you might find optics at 20 X 80 or even 25 X 100 which need to be attached to a tripod, say, for stable viewing. Lenses with a diameter of 100 millimeters come encased in binoculars so heavy that you'll usually find them bolted to a fixed support.

So there you have it. The essence of binoculars. Hopefully you'll have enough information now to at least decipher the basic marketing copy you'll find on the detail pages of binoculars you find offered for sale, whether here or on other sites.