Binoculars are optical instruments designed to magnify an object at a distance to a specified power and in a manner consistent with maintaining depth perception and a forward-facing stereoscopic field of view--essentially two telescopes aligned with each other to focus on the same object, rendering a magnified image for both an observer's eyes.
The advantage of binoculars over spotting scopes, also known as "field scopes", is in the wide viewing angle the bi-ocular design provides. When scanning for an object or looking at an object in motion (whether the observer, the subject, or both are moving), two telescopes are better than one.
Binoculars cover a larger area with their twin objective lenses, making it easier for the user to find, focus, and track an object. Our own biology confirms this: it is one of the reasons we have two forward facing eyes. As with telescopes, the greater the magnification required, the larger the optics for light gathering and focus power need be, as well as the housing for all that glass and mechanism.
For really powerful magnification over very long distance, binoculars can quickly become large, heavy instruments requiring a platform on which they can be mounted for use, unmanageable for an individual person to take on a hike. Even very powerful telescopes pose this problem at a relative scale. Binoculars are better suited from an engineering and usability compromise to lower powers of magnification for hand-held use.
Use binoculars for looking for and watching objects within a few hundred yards at low to medium magnification, telescopes on a tripod or other rest for studying immobile or very slow, predictably moving objects at great distance at higher powers of magnification.
Selecting a Pair of Binoculars
Binoculars, like any manufactured goods, vary in quality. The best binoculars are precision crafted scientific observation instruments that provide crisp, highly-detailed images magnified to the desired power over a wide range of light conditions.
Selecting a pair of binoculars for birding and bird watching, requires a bit of technical understanding. First and foremost to choosing a pair of binoculars is their ability to magnify and provide a bright image at a distance. One can easily assess this ability: a pair of binoculars will be marked with a simple multiplication figure such as "7X35" which will be physically printed onto the instrument. This is an expression of two important features of the binoculars.The power of magnification is expressed by the first number and the "X." In our “7X35” example, he binoculars would magnify the image to 7 times the size as seen by the unaided eye. In fact, seven or eight times amplification is considered ideal for viewing birds, as birders are likely to be relatively close to the birds they are watching because of local geography and lines of sight, trees and other natural and man-made structures reducing and obstructing more distant viewing.
A stronger magnification of up to ten times may be in order for watching shore birds, waterfowl, and high fliers, where obstructions are fewer and distances greater. Magnification powers higher than 8X do make keeping a clean focused image more difficult to maintain, as any shaking or movements on the handler's part will be amplified as well.
The second number is the diameter of the objective lens, usually addressed in millimeters. In our example this would make the objective lens 35mm in diameter. The larger the objective lens, the more light the binoculars are capable of gathering. More light gathering results in a brighter image and better visibility in less than direct bright sunlight. This is very important to a birder whose subject may be under the canopy of a tree or in the shade of a passing cloud, and whose identification may require a birder to tell slight differences in hues of plumage or subtle variations in markings and anatomical details.
The magnification power and objective lens diameter may be used in a mathematical formula for helping determine the brightness. By dividing the size of the objective lens by the magnification power, we can find the size of the “exit pupil.” The larger the size of the exit pupil, the better the image brightness. One should avoid an exit pupil of less than five millimeters, as the image will be too dim under less than bright direct light conditions.
There is more to brightness than a huge piece of glass cut and ground into a lens. It is all about the ability of the optics, the lenses, and prisms to pass the light that has been gathered through the instrument and into the eye of the observer. It is easy to determine the quality of brightness in a pair of binoculars: look through them in a well lit environment and see if the exit pupil is an even shining circle with clearly defined edges or if there is a darkening around the edges. Dimness or darkening around the edges of the exit pupil indicates a loss of light gathering.
High quality binocular manufacturers have a solution for this dimming of the light gathering properties of large lenses. They will coat all the glass surfaces with a non-reflective treatment to reduce the amount of light reflection off the surface of a lens or prism and allow more to be channeled through the glass.
We all know just how reflective glass can be...we see it off the windows of cars as blinding glare. These non-reflective coatings can have a great influence over the brightness of a magnified image in our binoculars. Anything less than fully coated means lost light and a dimmer image.
Some binoculars will only have the exterior surfaces of the lenses coated with this non-reflective material...these partially coated lenses mean a partial solution to light loss. Light can be lost inside the instrument as well as out.
The quality and precision of manufacturing and materials that go into a pair of binoculars have an enormous influence over the resolution of an image that can be obtained, and are typically reflected in the price.
Try a simple viewing test: Focus the binoculars on a map or other small printed poster hanging on a wall from about 25 feet away. You want the magnified image to be as crisp and legible as the subject throughout the entire field of view, with no blur or fuzziness in any part of the image. The better the binoculars the less likely you will be to find resolution an issue.
This brings us to field of view and what that means to a birder. The field of view is the width of the area you can see when looking through your optics. It is the "movie screen" of your magnified image. It is often expressed as either feet in width viewed at 1000 yards or in degrees as related to the trigonometry of the observer and the observed.
A wide angle of view is helpful in finding birds, but too wide an angle will result in poor resolution. Most birders prefer a compromised "standard" field of view over a wide field and the added distortion of the image that may result.
Binoculars really are two separate telescopes put together and this means it is crucial these two instruments are properly aligned to produce a clear, single image. If you can see double images through a pair of binoculars, they are out of alignment. This can be found in very inexpensive binoculars or in very good binoculars that have been exposed to excessive rough handling and banging around.
Be gentle with your binoculars, as re-alignment is a difficult task and should only be performed by a professional. Like any optical instrument, you should protect your binoculars from abusive handling and sharp impact.
As a final bit of information, we address those birders who wear eyeglasses. If you wear eyeglasses in your everyday life, you should leave them on when using binoculars. If you have to take off your glasses every time you want to see a bird magnified, you are in for missing more than watching. Also, with astigmatism you will be sacrificing the correction of your lenses for a less sharp image. Use the binoculars with the eye-cups up and your glasses on! The key to this is making certain the binoculars you use have long eye relief.
Eye relief is the distance the binoculars can be held from your eyes and still allow you to focus a sharp, bright image. Long eye relief is usually indicated as a feature by the manufacturer, so look for this.
After you have chosen a pair of binoculars and purchased them, remember to follow the manufacturer's recommendations with regard to maintenance and care. And enjoy the birds!