Designed to be compact and portable, binoculars from brands like Brunton, Bushnell and Vanguard essentially work just like two small telescopes mounted side-by-side. Every pair of binoculars has two eyepieces and two objective lenses. The larger objective lenses face forward and transmit light into the eyepieces, which magnify the image. Before the light hits the eyepieces, however, it first passes through two special prisms inside the binoculars, which correct the image. Without a set of prisms, the image would appear inverted.
The two most common types of binoculars have either porro prisms or roof prisms.
Roof prism binoculars use a series of silvered surfaces to reflect light, and don't require any offset. This allows roof prism binoculars to be smaller, narrower and lighter than most porro prism binoculars, which makes them appealing to people who prefer a less bulky design. Roof prisms also used fixed optical elements, so they rarely come out of alignment. The main drawback to roof prism binoculars is that light transmission is reduced slightly as it's reflected off multiple surfaces within the prism, which produces an image that is slightly dimmer than an image produced by a similarly sized set of porro prism binoculars.
Porro prism binoculars combine two Z-shaped prisms that are offset from one another by 90 degrees. Because of this offset, porro prism binoculars tend to be wider, with objective lenses that are offset from the eyepieces. One benefit of porro prism binoculars is that the objective lenses are spaced farther apart, which can add a greater sense of depth to the image. One disadvantage is that some porro prism binoculars can come out of alignment if they are dropped, and may need to be realigned by the manufacturer or an optics specialist.
Magnification (the first number in the binocular rating) is calculated by dividing the eyepiece focal length ratio by the objective lens focal length ratio. The most common magnification levels are between 8x and 12x. A magnification of 10x, for example, would cause an image to appear ten times closer. The higher the magnification, however, the more difficult it is to hold an image steady without a tripod. For this reason, most handheld binoculars don't exceed a magnification of 12x.
Objective diameter (the second number in the binocular rating) is a measurement of how much light can be gathered by the objective lens. A higher objective diameter produces a brighter, sharper image. Therefore, a pair of 8x42 binoculars will typically produce a brighter image than a pair of 8x22 binoculars. Of course, other factors, such as optical quality and special coatings can also impact image quality, so objective diameter shouldn't be your only consideration.
Field of view is typically noted as the measurement of width (usually in feet or meters) that is visible at a fixed distance (most commonly 1000 yards or meters). As the magnification increases, the field of view diminishes.
Exit pupil is the diameter of the image being projected by the eyepiece, and is usually measured in millimeters. Exit pupil is determined by dividing the objective diameter by the magnifying power. The most effective exit pupil measurement is one that is roughly similar to or larger than the diameter of a fully dilated human pupil. If the exit pupil measurement is smaller than the pupil, the image may appear dim or less clear. If the exit pupil measurement is too large, part of the image will be lost as it falls onto the retina.
Eye relief is the distance from the eyepiece to the optimal eye position, usually measured in millimeters, and may vary depending on the exit pupil diameter. Individuals who wear eyeglasses should consider binoculars that have adjustable eyecups, which will allow them to compensate for the added distance between their pupil, the eyeglasses and the eyepiece.
Other Binocular Features
Image Stabilization: Normally, higher magnification binoculars are difficult to keep steady without a tripod or monopod. Image stabilization incorporates a special mechanism, such as a gyroscope, to help minimize shaking in handheld binoculars with high magnification. However, binoculars with image stability are typically larger and heavier than binoculars without an image stabilization feature.
Optical and Anti-Reflective Coatings: Some binoculars feature special optical coatings to enhance image quality and light transmission through the prisms. Anti-reflective coatings, especially applied to the objective lenses, can also enhance light transmission and provide a brighter, clearer image.
Waterproof/Fogproof: Some higher quality binoculars are purged of air and filled with nitrogen or argon gases to enhance fog and water resistance. Other proprietary waterproofing treatments may add additional protection from condensation.
Just like binoculars, scopes have an objective lens, an internal prism and an eyepiece. Magnification, objective diameter, field of view, exit pupil and eye relief are typically measured the same way they are measured on a pair of binoculars (see the binocular section above for a detailed description of these aspects).
High-quality spotting scopes from brands like Pentax and Bushnell are excellent for wildlife observation, birding, hunting and long range target shooting when more magnification is preferred. Since spotting scopes typically have a higher magnification than binoculars, a tripod is required to provide a stable view. Some spotting scopes feature variable magnification. For example, a 20-60x80 spotting scope has an adjustable magnification between 20x and 60x, with an objective lens diameter of 80. Spotting scopes can include either a porro prism or roof prism design.
A rifle scope is similar to a spotting scope, although it is designed to be mounted to a firearm and takes the place of a rifle's iron sights when aiming. Rifle scopes from brands like Pentax and BSA Optics can either have a fixed magnification or adjustable magnification. Rifle scopes also incorporate some form of reticle, usually crosshairs, to assist with targeting. In addition to an objective lens, a prism (usually a roof prism) and an eyepiece, rifle scopes also include windage and elevation adjustment nobs. Some rifle scopes also include parallax adjustment.
Windage and elevation adjustments are typically used when sighting in a rifle scope, and may also be used to adjust for bullet drop when shooting at longer distances. Elevation adjusts the horizontal axis of the reticle, and windage adjusts the vertical axis of the reticle. Parallax occurs when the scope image (or target image) is not on the same optical plane as the reticle. In other words, if you don't adjust for parallax, an image may appear to exist on a slightly different optical plane from the reticle, which will become more apparent when you reposition your eye in front of the scope. This can lead to inconsistent accuracy. You should adjust for parallax when sighting in your rifle, although it may need to be readjusted if the magnification setting is changed on a variable magnification scope.
For an excellent video tutorial on how to mount and adjust a rifle scope, check out this video from the National Shooting Sports Foundation.
When used out in the field, binoculars and scopes will inevitably begin to gather dust and dirt. Skin oils and moisture can also cause smudging. When contaminants accumulate on objective lenses and eyepieces, it will eventually affect image quality. It's recommended that you use a brush, cleaning cloth and cleaning solution designed specifically for optics or cameras to clean the lenses. Do not use household glass cleaners or cleaning solutions.
If an optics cleaning kit is not available, you can use a cotton swab moistened with distilled water to carefully blot away dirt. Avoid scrubbing the lens with the swab, as this could cause small abrasions. Gently wipe the lens clean with a microfiber cloth. Wipe the exterior of your binoculars or scope clean with a moist towel. You can also use a compressed air duster similar to the kind used for computers to clean the focus wheel or nob crevices. Remember to use eyepiece and lens covers when your optics are not in use.