Article: Binocular Basics

Article: Binocular Basics

Time and Optics5/ 1/20

Binoculars are typically described by two numbers, such as 7x35, referred to as, “seven by thirty-five”. The first number given is the power or magnification. A 7x (seven power) binocular will make an object look seven times closer or seven times larger than you would see with the unaided eye.

The second number, 35, refers to the diameter or width (in millimeters) of the front, or objective lens. The size of the objective lens determines how bright an object will appear to your eyes. A third number commonly printed on binoculars is the angle or field of view. This number tells you how wide an area you will see.



The magnification of most handheld binoculars range from 6x to 10x. Seven and eight power models are considered to be the most versatile, multi-use binoculars. Al- though it seems sensible that a high power model would help you see things in more detail than a low power model, this is rarely the case. The largest drawback of higher power is that along with magnifying the object, it also magnifies the movement of your hands and body, which causes the image you see to shake or appear jumpy. Keep in mind that the shake will be noticeably worse during and for a short time after any physical exertion. A second drawback is that higher power models generally have a smaller field of view, causing difficulty in finding or following objects.

We do not mean that 9x and 10x binoculars should be avoided. Many experienced birders prefer 10x models, especially for birds that are difficult to approach, such as raptors and shorebirds. We suggest you try looking through a high power binocular to determine whether or not you are able to hold them steady. Handheld use of magnifications above 12x is extremely difficult, and we suggest using a tripod or window mount for the best results. When powers of 15x or higher are required to see detail at a distance, a tripod-mounted spotting scope should be considered.



A larger objective lens will gather more light and usually deliver a sharper, brighter image. We stress the word usually because once the light is collected, there are many factors that affect how well it is delivered to your eyes. These factors will be covered later in, “Optical Quality.”

The downside of a larger objective lens is that as the size of the lens increases, so does both the weight and the size of the binoculars. Binoculars that are too large or heavy to carry comfortably tend to get left behind.



Manufacturers use two interchangeable terms to describe the amount of area you will see while looking through bin- oculars. Linear field of view is the most commonly used method and describes how wide an area (in feet) you will see while viewing an object one thousand yards away, usually denoted as xxx feet at 1,000 yards. The second method, used by most European manufacturers, is angular field of view which describes the same area measured in degrees of arc. Each degree is 52.5 feet across at 1,000 yards away, so to convert the angular field of view into linear field of view multiply the degree of arc by 52.5 feet. As a general rule, the higher the magnification, the smaller the field of view. Therefore a 10x model will usually have a smaller field of view than a 7x model. In addition to magnification, the de- sign and quality of both the prisms and eyepieces also affect the field of view. Although a wide field of view is desirable, beware of inexpensive models boasting a field of view of 10 degrees (525 feet at 1,000 yards) or more, because sharp- ness is usually sacrificed for an extremely wide field.


Beyond the Basics


You can see the exit pupil of any binocular by holding it at arm’s length and pointing it at a light. The exit pupil is the circle of light leaving the eyepiece. Under dim and dark conditions a larger exit pupil will provide a brighter image. How- ever, on bright or sunny days the pupils in your eyes close down and will not make use of all the light. To determine the exit pupil size (diameter in millimeters), divide the diameter of the objective lens by the magnification. A 7x35 model will deliver a 5 mm exit pupil (35÷7=5). The exit pupil of most binoculars varies between 2.5 mm and 7 mm.

The size of the objective lens directly affects the brightness of the image and the physical size and weight of the binoculars. It does not affect the field of view or area that you will see.



The term close focus indicates the shortest distance (in feet) you can be from an object and still maintain a sharp focus. For uses such as bird-watching or nature studies, binoculars with a close focus of 12 feet or less are recommended. Models with a close focus of 8 feet or less are considered exceptional.



Eye relief refers to the distance (in millimeters) your eyes can be from the eyepiece and still see the entire field of view. Although short eye relief poses few problems for those who do not wear glasses, long eye relief is important to people who need or want to wear eyeglasses or sunglasses while using binoculars. We suggest an eye relief of 15 mm or longer if you wear glasses and want the full field of view.



Four major factors determine the optical quality of bin- oculars; optical alignment, prism quality, lens quality and coatings. A durable and rigid housing is necessary to insure the precise optical alignment of all lenses, prisms and eye- pieces. If alignment is off by a small amount, the muscles of your eyes must work to compensate, which can cause eyestrain and headaches.

Double or crossed vision will occur if optical alignment is off by a large amount. The prism quality depends on both the quality of glass used and prism design. Most high quality binoculars use BaK-4 prisms which provide crisp colors and clear, round exit pupils. Lower quality prisms such as BK-7 limit sharpness and detail. They also dull colors and provide an exit pupil with squared off edges. Color fidelity, brightness, contrast and sharpness are all affected by lens quality. High quality glass such as ED (extra-low dispersion) HD (High Definition) or fluorite are used to provide the sharpest and most brilliant images possible. Anti-reflective coatings significantly improve brightness and contrast while reducing both glare and light loss. Fully multi-coated optics will provide higher contrast and detail than multi-coated, fully-coated or non-coated lenses.

Modern binoculars use one of two prism designs: porro or roof. The diagram (top right) shows the path that light follows through porro and roof prism models. Utilizing fewer prisms and a simpler light path, porro prism binoculars are less costly to manufacture and usually offer optics equal to more expensive roof prism models. Many premium quality binoculars use the roof prism design because they are more compact and will tolerate rougher use.



Three basic focus systems are used: center, individual and permanent focus. The center focus style is most common. Both sides focus simultaneously by turning a central dial. This allows for quick and simple focusing while view- ing moving objects. Binoculars that are focused by turning each eyepiece individually are best suited for viewing slow-moving or stationary objects that do not require constant refocusing. Permanent focus or non-focusing systems produce inferior detail and are used mainly in lower quality models. Permanent focus (focus free) models have two major drawbacks: close focus is normally 30-40 feet, and they lack a diopter which allows you to adjust for differences in eye strength.



Binocular manufacturers use many terms to describe how wet a particular model can get without internal fogging or damage. Only models labeled waterproof are guaranteed against water leakage and are often filled with nitrogen to prevent fogging. All other models vary greatly in their ability to resist moisture. Descriptions such as weather resistant, showerproof, splashproof, water resistant, and weatherproof usually indicate a model is highly resistant to moisture, but generally offer no guarantee against water damage or fogging under extreme conditions. Rubber armoring does not affect a binocular’s resistance to water.



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