Binoculars, not to be confused with a monocular, are an optical tool built to enhance the vision of both of the eyes we have on our face, whereas, monocular are for just one eye at a time. A binocular could be described as two monocular joined together with a hinge and most often sharing a common focusing structure. A binocular is transmitting the light of an image to both eyes simultaneously providing a more real-life perception of depth and distance. Binoculars choices range from a few dollars to many hundreds and even thousands of dollars depending on many factors.
Your first consideration in choosing a binocular might be:
Binocular Prism Systems
The prism system of a binocular bends and reflects the light so as to reduce the distance needed to contain an otherwise long optical path. This allows for the familiar binocular designs, as opposed to trying to hold up to both eyes long telescopes full of magnifying glass pieces from one end to the other. The prism system of a binocular also turns the naturally occurring upside-down magnified image, right-side-up. There are two types of prism systems, roof and porro.
In roof prism binoculars, the prisms overlap closely, allowing for the more compact design with the objective lenses (front lens) in line directly with the eyepiece lens. The more compact design of roof prism binoculars, results in a slimmer, more streamlined shape in which the lenses, objective (front lens) and eyepiece, and prisms are in a straight line. Roof prism binoculars may cost more because of this more complex design, but, are less bulky, smaller more compact, than an equivalent porro prism binocular model.
In porro prism binocular design, the objective lens (front lens) is offset wider than the eyepiece lens. Porro prism binocular designs provide a greater depth perception. This is because the binoculars objective lenses (front lens) are wider apart than in a equivalent roof prism binocular design. This binocular design is gathering the image from a wider angle, giving a better perception of the depth and distance of the image in view. This binocular design also generally offers the image with a wider field-of-view (the distance in view from the far left to the far right of the image). Because of the more simplified construction of a porro prism binocular system, you should find some of the best consumer values.
Be sure to choose binoculars from optical prism systems made from boroscillicate (BK-7) glass or barium crown (BaK-4) glass. BaK-4 (barium crown) is a higher quality glass yielding better brighter images and better edge-to-edge image sharpness.
Your second consideration in choosing a binocular might be:
Binoculars models are often referred and compared to each other by a two number set with a lower case letter "x" in between the two numbers. For example, 8 x 35, the first number is the power of magnification of the binocular. If the first number of the number set is 8, the object being viewed appears to be 8 times closer than you would see it without using the binocular. If the first number is 10, 10 times closer, and so on. Although many binocular models come in 12, 16 and 20 power magnification and zoom binocular models allow infinite adjustment of power up to that binocular models top end value, it is important to understand more is not always better. Any binocular beyond 10 power magnification can be difficult for the human hand to hold steady enough to see much detail of the object being viewed. Users can enhance the stability of the image by changing from a full standing position while viewing to a viewing position where you can rest your elbows on a table, bench or even your vehicle?s roof, hood, trunk. Stability of the image may be less important when viewing landscape, mountains and waterfalls. The value of a stable image increases when viewing smaller objects such as animals. Being able to distinguish different markings and features of an animal may require the higher power binocular models and stability of the image. Stability of the image can be increased by using a tripod with your binocular. If you plan to use a tripod, be sure to choose a binocular model with the tripod adaptor built in. Many binocular models have this feature.
The second number in the two number set describing binoculars, 8x35, is the millimeter diameter of the objective (front) lens. The larger the objective (front) lens, the more light that enters the binocular, therefore, the more light that eventually gets to your eyes. The more light allowed to your eyes, the brighter the image. Choosing a larger objective (front) lens will enhance viewing in night time, dusk, dawn, in a forest, shadow of a mountain or other low light conditions. The smaller objective (front) lens found on roof prism binocular models are more than adequate for full sunlight conditions. The number defining the exit pupil is more directly related to the amount of light allowed to be visible by your eyes. The exit pupil number is simply the objective lens (front) lens number divided by the power of magnification number. For example, 8 x 32, 32 divided by 8 equals 4, exit pupil is 4. The exit pupil number is the size of the circle of light visible at the eyepiece of the binocular. The larger the exit pupil number of the binocular is, the brighter the image in view.
The last number to understand when purchasing binoculars is ?Field of View? (FOV). This is the edge to edge or side to side distance in feet and meters of the viewing area. This measurement is given as the width of view at 1000 yards (ten football fields) away. For example, 300 foot FOV would mean your image at 1000 yards (3000 feet) away is also 100 yards (300 feet) wide and tall since FOV is the diameter of the circular viewing area. FOV may also be expressed as angular measurement in degrees, using the constant of 52.5. For example, 300 feet at 1000 yards equates to 5.714 degrees. If the degree of FOV is known but not the feet at 1000 yards you can multiply 5.714 time 52.5 to calculate 300 feet FOV at 1000 yards. A binocular with a larger wider FOV is better for following movement or action. Generally, as the magnification power increases the FOV decreases.
All other considerations in choosing a binocular:
Functions and features
Chemical surface coatings on the lenses and prisms reduce light loss from glare and reflection for a brighter, higher contrast image with less strain on the eyes. Four definitions of coated optics are:
Coated - A single layer on at least one lens surface. Fully Coated - A single layer on all the external lens surfaces exposed to air and the elements. Multi-Coated - Multiple layers on at least one lens surface. Fully Multi-Coated - Multiple layers on all external lens surfaces exposed to air and the elements
Some binoculars are O-ring sealed and nitrogen-purged for total waterproof and fogproof protection. These binocular models can withstand complete immersion in water and stay dry inside because of the O-ring seal. The interior optical surfaces won't fog due to rapid temperature change or humidity because of the nitrogen-purged and sealed construction.
Eye relief is the distance a binocular can be held away from the eye and still present the full field-of-view. Extended or long eye relief reduces eyestrain and is ideal for eyeglass wearers.