In porro prism binoculars, the objective or front lens is offset from the eyepiece. Porro prism binoculars provide a greater depth perception and generally offer a wider field-of-view. Because of the simplicity of this system, some of the best values can be with a porro design.
On any big game hunting trip, and on many outings for small game and waterfowl, next to your weapon of choice, a good pair of binoculars may be the most important piece of equipment you bring. Although most hunters today have scopes on their rifles, it is simply irresponsible and dangerous to use a scoped rifle to glass something you see off in the distance. Remember, there's a rifle sitting underneath that scope, and what you are pointing at could be another person. Don't do it. There's simply no need for that when there are so many good binoculars available today at reasonable prices. But since there are so many different models of binoculars to choose from, selecting the right one for you may seem difficult. Here's what you need to know, including some of the terms used in the optics industry and what they mean for you.
Binoculars come in a wide variety of sizes, magnifying powers and features, but they all utilize either a Porro prism system or a roof prism system, which refers to the type and configuration of the internal prisms used to magnify and transmit light through the binocular to the eye. Porro prism binoculars can be recognized by the fact that the front or objective lens is offset from and not in line with the eyepiece or ocular lens. Porro prism binoculars can provide a slightly clearer, more three-dimensional image with greater depth perception, and generally offer a wider field of view (F.O.V.), or the actual width of the sight picture at a specific distance. Due to the relative simplicity of the Porro prism system, they generally cost less than similar roof prism versions, however, they tend to be bigger, bulkier and heavier.
In roof prism binoculars the internal prisms overlap closely, allowing the objective lenses to line up directly with the eyepieces. The result is a slim, streamlined shape, with less bulk and perhaps a bit more ruggedness than Porro prism designs. For these reasons, roof prism models tend to be a bit more popular with sportsmen. If this style is your preference, look for a model with phase correction, which is a feature that prevents interference when the path of light crosses over itself while being reflected off of the various surfaces of the internal prism.
In roof prism binoculars, the prisms overlap closely, allowing the objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms are in a straight line.
You will notice that the specifications of a typical binocular are usually stated as 7x35mm, 8x40mm, 10x42mm, or some variation thereof. The number before the "x" refers to the magnification, which means that the object will appear that many times closer or larger than it actually is. Although a higher magnification will mean a better look at the object, the higher the magnification, the smaller the field of view, which can make scanning for game more difficult. Most hunters prefer 8x or 10x, which provides a good balance of magnification and field of view, but some will go as high as 12x. Anything more than that will not only have a small field of view but will likely be too difficult to hold steady without the aid of a tripod or other support, as the higher power will magnify the unsteadiness of your hands to the point of making prolonged viewing nearly impossible. The vast majority of binoculars are fixed-power, whereby the magnification is permanently set at a given level, but a few are variable-power, which can be adjusted or zoomed within their set range.
The number following the "x" refers to the diameter of the objective lens, expressed in millimeters. The larger it is, the larger the field of view will be at any given magnification setting, but also the more light the binoculars will gather and brightness is one of the most important features to look for in binoculars. High-quality binoculars have the ability to gather available light in through the lenses and utilize it in such a way that you can actually see better during low-light conditions while looking through the binoculars than you can with the naked eye. The benefit of this to hunters is obvious. The key term here, however, is high-quality, but what makes a high-quality binocular? Unfortunately, large objective lenses do not automatically translate into a bright image.
A binocular with 40 mm objective lenses can actually be brighter than one with 56 mm lenses. It all depends on the quality of the glass used, the overall construction of the unit, and particularly on the coatings used on the internal glass. In order to reduce glare and the amount of available light lost during transmission from the object to your eye, special chemical coatings are applied to the surface or surfaces of a lens. The quality, number and position of these coatings determine how much light is transmitted. Here are the options available and what they mean:
Coated -- a single layer is applied to at least one lens surface
Fully-coated -- a single layer is applied to all air-to-glass surfaces
Multi-coated -- multiple layers are applied to at least one lens surface
Fully multi-coated -- multiple layers are applied to all air-to-glass surfaces
As mentioned, the overall brightness, as well as the sharpness and clarity, of a pair of binoculars depends upon a lot of factors, but one way to compare the inherent brightness from one model to another is to compare the diameter of the exit pupils. This refers to the size of the circle of light visible at the eyepiece of a binocular, when pointed at a light source and held about a foot away, but really means how much light is available to the human eye. The larger the exit pupil measured in millimeters, the brighter the image, everything else being equal. To determine that number, divide the objective lens diameter by the magnification (an 8x32 model has an exit pupil of 4mm). Full-sized binoculars should have exit pupils at least in the 4-5mm range. Anything larger than that is typically larger than the pupils of an adult human's eyes, meaning that there is more available light than the eye can use, at the expense of lugging around bigger and heavier lenses. Anything smaller than that and the image is likely not as bright as it could be.
The Dioptre Adjustment is a "fine focus" adjustment ring usually located around one eyepiece to accommodate for vision differences between the right and left eyes.
The exception to the 4-5mm exit pupil rule is in regard to compact or pocket-sized binoculars. These models are usually 7x or 8x magnification, with objective lenses usually less than 30mm in diameter. As a result, they tend to not be as bright as full-sized models, but this is offset by the convenience of a small, lightweight binocular that you can carry in your pocket or in your glove box. A quality compact binocular that is always with you when you need it is far better than a heavy, bulky model that you always leave back at camp or in your vehicle.
In terms of cost, top-quality binoculars are not cheap. However, perhaps more so than with any other piece of your hunting equipment, you do get what you pay for when it comes to optics. Expect to pay anywhere between $200 and $2,000 for quality binoculars. The rule of thumb when it comes to optics is to buy the best that you can afford. You won't regret it. Not only are high-quality optics clearer and brighter, but they won't cause headaches or eye fatigue from hours of glassing the way that cheap optics can, and they will last several lifetimes.
What else do you get for this money? Many binoculars are rubber-armored, to provide some shock absorbing protection against scratches and bumps in the field, and it also makes them quieter to carry and more comfortable to hold. Top quality models are waterproof, fogproof and shockproof, through the use of O-ring seals and nitrogen gas filling or purging.
Many models also offer a dioptre adjustment, which is a fine focus adjustment ring usually provided around one or both eyepieces, rather than just the standard center focus wheel, to accommodate for vision differences between the right and left eyes. Other features such as roll-down or twist-down eye cups for eyeglass wearers, lens caps and neck straps are common on most models.
Now that you are armed with this knowledge, the best way to actually pick a binocular that is right for you is to try as many as you can in your price range. Examine each one and look through them all, preferably at distant objects, for a few minutes each. If you start to feel eye strain or fatigue, try another pair. Everyone's eyes are different, so you may have to try a few before you find one that feels good in your hands, offers that perfect level of brightness, sharpness and clarity, in a package that is not too big, heavy or hard on the pocketbook. Once you find it, you'll wish you'd done it sooner.
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