Binocular Buyer's Guide and Glossary - What makes up a good set of glass.

Author: Mark Mazour

One of the handiest things to have in the outdoors is a quality pair of binoculars. Whether you are hiking, viewing wildlife, hunting, or even at a sporting event, binoculars can bring the outdoors a little closer to you.

Binoculars come in handy to survey the terrain ahead, scout for and view wildlife without spooking them, or just enjoy a piece of nature up close that you may have otherwise missed. Starting off by selecting the right pair is crucial to your enjoyment of the outdoors.

Basically, binoculars do two main things for you. They increase the size of the image you are viewing, and they let in more light than your eyes can, making images appear brighter in low light conditions. When you first start looking for binoculars, it can be confusing comparing one model to another.

Power

The first main comparison used in binoculars is power. Binocular powers are expressed as two numbers such as 8 x 42. The first number, 8 in this example, is the magnification. It expresses the magnification as a factor compared to the naked eye. So an 8 power magnifies the objects in view up to 8 times. An object would appear to be 8 times closer than it would with the naked eye. Therefore, a higher number has a greater magnification.

The second number, 42 in this example, is the diameter of the objective lens in millimeters. So a 42 designation means that the outer lens is 42 mm in diameter. A larger number indicates a larger lens. Large lenses are more bulky, but they also let in more light, making your image clearer - especially in low light conditions.

Field of View (FOV)

FOV @ 1000 yards is the second most used comparison with binoculars. What this means is how wide of an area (in ft.) that you can view through the binoculars at 1000 yards. A higher number indicates a wider area, while a smaller number indicates a narrower area. The focal length of the objective lenses and the eyepiece design has the most impact on the actual FOV. The power of the binoculars also has an inverse relationship with FOV. As the magnification increases, a smaller FOV results.

Your choice of FOV depends on your individual use of the binoculars. If you are using them in a wide-open area to scan for mule deer, a narrower field of view is not a big deal. However, if you are scanning a dense forest for hidden black-tailed deer at ranges at or around 100 yards, you will want to select a model that has a wider field of view.

Exit Pupil

Exit pupil is related to the power of the binocular and the size of the objective lens. If you hold a pair of binoculars away from your face, you will see a small circle of bright light in the eyepiece. This is the exit pupil, the size of the beam of light that leaves the binocular. The exit pupil diameter can easily be calculated (in mm) by dividing the diameter of the objective lens by the power. Therefore, a 8x42 binocular has an exit pupil of 5.25mm. On a bright day, the human pupil will vary from 2mm at noon to 4mm later in the day. When your eyes become adapted to dark conditions, the pupil will vary from around 5mm to a maximum of 9mm.

In daylight, having a binocular with a larger exit pupil will have little effect. The only difference you may notice is that you will be able to move a binocular with a larger exit pupil and still maintain the image, which is extremely helpful if you are in unstable conditions, such as in a boat. The main difference occurs in low light conditions. If you plan on using your binoculars near dawn or dusk, which is a time when many hunters depend on their optics, it is recommended that you select an exit pupil greater than 4mm, to fully take advantage of the amount of light your eyes can let in. An exit pupil larger than your pupil’s diameter at the time does not result in a brighter image. Your eye can only handle so much.

You can see from the exit pupil calculation that if you choose a higher power binocular, you will also need to increase the size of the objective to maintain the same diameter of light leaving the binocular.

Prisms

As light passes through a pair of binoculars, the image becomes inverted. Erecting prisms are used within the binoculars to correct this problem. The two main types you will find in binoculars are Roof and Porro Prisms.

• Roof Prism – A roof prism design reflects the light 5 times, and the resultant light comes out on the same line that it came in on. This straight through design lends itself to slimmer dimensions, a more compact body, and usually, lighter weight, as the objective lens is in direct line with the eyepiece. In general, roof prisms cost more, due to the difficulty of manufacture.
• Porro Prism – A Porro prism binocular requires a bit larger body than a roof prism as the light is only reflected 4 times, and it comes out on a different line than it enters the objective. From a side view, the objective lens is usually a small distance above the eyepiece.

Many people debate about which design is better, and it actually depends more on the glass quality, manufacturing tolerances, and individual design as to which delivers the best image. Within Porro prisms, two different types of glass are used, BK-7, which utilizes boro-silicate glass, and BAK-4, which uses barium crown glass.

The BAK-4 is a finer glass (higher density) and eliminates internal light scattering, therefore producing sharper images than the BK-7 glass. The finer glass in a BAK-4 prism comes at a higher price than ones using BK-7 prisms.

Optical Coatings

The largest limitation of light transmission in binoculars is reflected light. Any time that light strikes a glass surface, up to 5% of the light can be reflected back. In a pair of binoculars, light can pass through at least 10 different glass surfaces. If you do the math, you can see that it is easy to lose 50% of the ambient light. Plus, much of this reflected light remains inside the binocular causing glare and poor contrast.

However, if a thin chemical film (a common one being Magnesium Fluoride) is used to coat the surface of the glass, much of the reflection can be eliminated. The coating reduces light loss and glare, increases light transmission and results in brighter, clearer images. By coating a surface with multiple films, the effect of the coating is increased, at times limiting the amount of reflected light to between 0.25% and 0.5% per glass surface.

The coating must be applied correctly and uniformly, or the effect is lost. You may also notice that certain coatings exhibit different colors. Contrary to popular belief, this does not have a direct effect on the quality. Depending on the chemicals used and number of coatings, colors can vary from violet to blue, to even red, yellow or green.

While any coating will usually increase the performance of a binocular, you need to understand several different definitions to make a full comparison or justify the expense of a certain model.

• Coated Optics — Generally means that one or more glass surfaces on at least one lens have received an anti-reflective optical coating.

• Fully Coated — Generally means that all glass surfaces have been coated with an anti-reflective optical coating.

• Multi-Coated — Generally means that one or more glass surfaces on at least one lens have received multiple anti-reflective optical coatings.

• Fully Multi-Coated — Means that all glass surfaces have received multiple anti-reflective optical coatings.

• Phase Shift Coating — A phase shift coating is used on the Roof Prism of many newer models to correct for light loss on the horizontal image plane. As light waves come through a Roof Prism, up to 70% of the waves reflected off one roof surface will be a wavelength shifted from those coming off the other roof surface.

Okay, what does that mean? What can happen is that the phase difference between the horizontal and vertical light in a non-coated Roof Prism can cause a loss of contrast. A phase shift coating in upper-end Roof Prism models corrects this, increasing contrast and image quality, putting them on equal playing fields with high end Porro Prism models.

While phase-coated Prisms will provide more contrast than a non-phase-coated pair, the small phase error on non-phase-coated roof prisms can only be detected if all other manufacturing tolerance errors are minimized. In other words, phase coating a cheap pair of Roof Prism binoculars does little to improve the final image.

Eye Relief

Eye relief is the comfortable distance that a binocular can be held from the eye, while still allowing the viewer to see the entire image. A long eye relief allows for comfortable viewing and is a must for viewing with eyeglasses or sunglasses. Another option to look for is extendable eyepieces or fold down rubber eyecups. These features will allow you to obtain a larger field of view while still keeping your glasses on.

Other Considerations

Some other considerations to be aware of are the size and weight of binoculars. While a high powered set of full-size 10 x 42 binoculars may give you some serious range and let in a lot of light, they can weigh almost two pounds or more. The weight will vary with the amount and type of glass used within the binoculars, but in general as power and the size of the objective lens increases, the weight does as well.

If you are packing in a ways, a big pair of binoculars can weigh heavily on your neck or in your pack. Most compact binoculars weigh under a pound and can fit in a shirt pocket, but they do not let in as much light and can be a bit more straining to the eye.

If you’re active in the outdoors, two final things to look for are rubber coating and waterproofness. Not only does the coating make your binoculars non-slip, even when wet, but it softens the blows of hard use and protects your binoculars for the long haul. An added bonus for hunters is the rubber coating also makes your binoculars quiet and non-reflective for that stealthy approach.

Another big concern, if you count on your binoculars in all kinds of conditions, is waterproofness. Some less expensive models are not waterproof, and that will be a concern if you are using them in wet weather. Other models are fully sealed and charged with nitrogen gas to keep them fully waterproof - allowing for an occasional dunking into the lake.

Cost

Cost is probably the largest differential factor in purchasing binoculars. They come in many models and many price ranges, and they all have their place. There is no substitute for high quality glass in providing high quality images, and good glass and good coatings cost good money - period. More expensive models are usually constructed to tighter tolerances and have better glass with more coatings.

However, that does not mean that you must spend $900 to have a decent set of binoculars. How much you spend should also depend on how often you use your binoculars. Some professional guides, whose success depends on glassing for animals, spend much of their day looking through their binoculars, and they opt for the highest quality optics to make their job easier and avoid eye strain.