Glossary of Binocular Terms
Magnification
Generally, the range in magnification for hand-held binoculars is from 6 to 10 in power. In a binocular designation (7x35, for example) the first number indicates the magnification, or how much larger, or closer, the object will appear than seen with normal vision. When considering magnification, more is not necessarily better. As magnification increases, brightness and clarity may diminish, depth of field can become shallower, and the field of view is usually more restricted. More noticeable and disturbing at higher powers are fine hand tremors and the effects of atmospheric conditions, such as the distortion caused by heat waves.
If your bird watching is done primarily at close range, such as in woodland areas or in your backyard, then a good 6, 7 or 8 power binocular might be the best choice. This range of magnification generally gives you a larger picture (wider field of view), which is especially important for viewing objects relatively close at hand. Also, binoculars of this magnification usually deliver better performance under conditions of low available light, due either to the time of day, weather conditions, or shadows caused by dense vegetation or other structures. This same type of binoculars also work well for fast-moving action like sporting events, since the wide field of view allows the action to remain in the viewing area.
For long-distance bird watching or where greater detail is required, a higher magnification of 8, 9 or 10x should be considered. For example, the demands of observing in wide-open terrain with little cover are best met with a binocular of 9 or 10x. This generally holds true for situations where there is a need for critical field mark identification, as in observing raptors and shorebirds or when the object or animal is difficult to approach. Magnification, as a binocular parameter, should be considered as it relates to other factors such as aperture size, exit pupil, hand-held stability, atmospheric conditions, available light, optical design, and the weight of the binoculars.
Eye Relief for Use with Glasses
Eye relief refers to the distance behind the binocular eyepiece at which the whole field of view is clearly visible to the observer. With binoculars, this distance will vary from as little as 5mm to as much as 23mm and it is an important specification for those who prefer or need to wear eyeglasses or sunglasses. Unless a pair of binoculars have a minimum eye relief of about 15mm, there will be some difficulty in comfortable observing and in seeing the complete field of view when wearing eyeglasses. If an individual's eye correction is for near or far-sightedness then the binoculars focusing travel will usually accommodate for this without the need for wearing the eyeglasses. However, for moderate to severe astigmatism it is almost a necessity to wear eyeglasses for serious observing.
Most modern binocular models have fold-down rubber eyecups so that eyeglass wearers can bring the binoculars in closer to their eye, which improves viewing significantly. Some models are particularly designed to give long eye relief (sometimes called "high eye point"). Binoculars with long eye relief usually have long rubber eyecups so that you can also use them comfortably without eyeglasses. Generally, binoculars with eyepieces designed for long eye relief will not have a particularly wide field of view, so a decision has to be made as to which feature is most important.
Weight
In general, the weight of a pair of binoculars goes hand in hand with their size. Larger binoculars with big objective lenses weigh more but gather more light. Smaller binoculars weigh less but the images aren’t as bright in low light situations. For many people, the weight of binoculars is a concern. Try to balance the light gathering needs your use dictates against the comfort of long-term viewing. Many people choose middle weight binoculars as a compromise.
Objective Lens Size
The second number of a binocular designation refers to the diameter, in millimeters, of the front, or objective lens. The diameters usually range from 20 to 50 millimeters and this number will almost always be directly related to the size of the binoculars. So called "giant binoculars", used mainly for astronomical purposes, may have up to 70 or 80mm objectives, whereas compact models will usually be 20 to 25mm in diameter. The objective lens size, or aperture, determines the amount of light that will enter the optical system. The common assumption that the size of the objective lens will determine the field of view is seldom true as field of view is controlled largely by the optical design of the binoculars.
A larger objective lens will gather more light and theoretically provide greater detail and clarity of the image. This is especially true under low light conditions. Because the amount of light that will enter the objective lens will vary by the square of the change in the radius, a small difference in objective lens size will have a greater impact on the light gathering ability than one might first suspect. Once the objective lenses gather the light into the binoculars, other factors determine how much light is transmitted through the optical system and all of these factors, including the aperture, combine to determine the brightness and clarity of the image you actually see. These other factors include magnification, exit pupil size, eye pupil size (controlled largely by the amount of available light), the presence and type of antireflection coatings used, and the size and quality of the optical glass and prisms used in the construction of the binoculars.
Prism Type and Design
Porro Prisms
Binoculars use image-erecting prisms to provide correctly oriented images. The two basic designs used in optical systems are the Porro prism (off-set barrel) and the roof prism (straight barrel). Another modification of the Porro design is the reverse Porro prism, commonly used in compact binoculars to reduce overall size.
Porro prisms have been traditionally larger and heavier than equivalent roof prism binoculars. With recent modifications in optical design and production methods, modern Porro prism binoculars are much smaller and lighter in weight and may actually be lighter in weight than a roof prism of the same configuration. The Porro design is capable of delivering a wide field of view with excellent image sharpness. Also, they can usually be purchased at almost half the price of a comparable quality roof prism model.
Roof Prisms
Roof prisms are generally smaller and more streamlined in size and able to tolerate rough treatment better because of their compact optical design. Most models have an internal type focusing mechanism which gives them a high degree of structural integrity, making them less susceptible to internal fogging and to potential dust and moisture entry. Their complex prism configuration and the precise tolerances required during manufacture make top-quality roof prism binoculars significantly more expensive than those of Porro prism design.
The optical design of a particular binocular model is a compromise of several parameters to limit the amount of certain aberrations inherent in any optical system. The quality of glass used in the binocular lenses and prisms will determine the light transmittance, color fidelity, and clarity of the image. The size and design of the prisms will affect sharpness, with quality optical glass delivering clarity from edge to edge of the field of view (sometimes called a "flat field"). Binoculars that use prisms made of costly, high density glass (such as the BaK-4 type) will provide clear, circular exit pupils.
Collimation is the mechanical alignment of the optical elements in a pair of binoculars. Both right and left hand optical axis must have proper orientation and location within the binocular barrel and must be parallel to each other in a quality optical system. High-quality mechanical construction will ensure that the lenses and prism blocks maintain their correct alignment and provide years of comfortable viewing without headaches or eyestrain.
Anti-reflection coatings reduce light loss and glare due to reflection of light at air-to-glass surfaces. Besides increasing the light transmission, coatings also provide sharper and higher contrast images and aid in detailed identification of low contrast subjects in limited available light. Multilayer anti-reflection coatings (multicoatings) provide a significant increase in light transmission over single layer coatings and are used in all high-quality optical systems.
These are among the most important factors concerned with the optical quality and construction of binoculars. A poor optical instrument will not reproduce the image clearly and with frequent use may cause eyestrain, headaches, and fatigue. By investing in high quality binoculars, you will benefit with years of comfortable and enjoyable viewing.
Close Focus
The close focus numbers used are averages because there is no exact way of measuring this in binoculars. Because everyone's eyes are different, and the travel of a focus wheel is fixed, the distance you can reach on the near end varies with the user. Generally, most folks are within a foot of the numbers we provide here. Those most interested in close focus under 10 feet are butterfly enthusiasts.
The ability of binoculars to focus closely is a much desired characteristic for many situations such as critical identification of birds in the field or observing the intricate detail of insects or plants. The close focus distance of a particular model depends on its optical design and to some extent on the individual's own eye characteristics. A minimum distance of 18 to 20 feet is usually acceptable but many birders prefer models that are capable of a close focus of 8 to 13 feet.
Waterproof
Waterproof binoculars are literally submersible and cover water damage under warranty. This is a great idea for boaters, hikers, hunters, and birders that work in all weather conditions, or might drop binoculars overboard. Many waterproof binoculars are also purged with nitrogen gas to prevent interior fogging in temperature extremes, high humidity, and rapid elevation change. Waterproofing binoculars is an expensive part of the manufacturing costs so make sure it is something you really need. When comparing binoculars of the same price that are pretty much equal except for the waterproofing, the non-waterproof models typically have superior optics.
Price
The price of binoculars is a good indicator of their overall quality. Generally speaking, the more money spent on a pair of binoculars, the better the optics and durability. As with all things, there is a point of diminishing returns. When comparing binoculars from the $100 price range with models in the $300 level, there is a very noticeable difference in optics and construction. When comparing the same $300 binocular with models costing $1000, you will see differences, but not as much as in that first jump in price.
Field of View
When looking through binoculars, the widest dimension of circular viewing area that you can see is described as the field of view. This is usually measured either in terms of linear feet at 1000 yards or in terms of angular degrees. Each degree of field corresponds to 52.5 feet at 1000 yards and binocular fields of view will generally range from 5 degrees (263 feet) to 11 degrees (578 feet). As a general rule, the field of view will decrease as the magnification increases so 10-power binoculars will usually have a smaller field of view than 7 power. Field of view, however, is mainly determined by the optical design of the ocular (eyepiece) and is rarely a function of the size of the objective lens.
For observing at close quarters in deep woods, scanning the sky for raptors or large flocks of migratory birds or for picking up fast moving objects, a wide field of view is desirable. Wide-field binoculars, however, tend to be heavier and bulkier than binoculars with standard fields (because they employ larger prisms and eyepieces) and on many models there is a noticeable loss in sharpness at the edge of the field. Also, if you have too large a field (9 to 11 degrees, for instance) the object you want to observe may become lost amid the confusion of its surroundings. Another problem is that people who need to wear eyeglasses when viewing will usually have difficulty seeing the complete field with wide-angle optics. Wide field binoculars are generally the most popular for nature observation but you should consider all these factors in your evaluation of this feature.
Depth of Field
This parameter refers to the distance from "near to far" that is in focus at a certain setting of the focus adjustment or at a certain distance. In a given system, as the magnification increases, depth of field decreases. This fact is one of the disadvantages of observing with high magnifications and why depth of field is usually more important in comparing spotting scopes or telescopes than binoculars. At very high magnifications, the depth of field can be so shallow that precise focusing is critical and so the location, size, action, and feel of the focusing adjustment is an important consideration. Depth of field also changes with the distance observed, usually decreasing in depth as the distance decreases.
Transmittance
As light travels through binoculars, a certain percentage of that light is lost through absorption and reflection at each air-to-glass surface or inside the prism system itself. The amount of original light available to the observer by the time it exits the eyepiece will vary from as low as 50 percent to as much as 97 percent, depending on the quality and number of optical glass elements used in the lenses and prisms, configuration and size of the prisms, collimation of the optical system, and type and amount of anti-reflection coatings present. This is an important factor that directly affects the actual brightness of the observed image. The term used to describe this percentage of light that is not lost through the optical system is transmittance, and for most quality binoculars this figure will usually be above 90 percent. With this factor taken into account, it's possible for 10x40 binoculars (exit pupil 4mm) with a high transmittance (90 percent) to actually deliver a brighter image than 7x35s (exit pupil 5mm) with a lower transmittance (70 percent).
Exit Pupil
The exit pupil is the magnified image in the eyepiece as it leaves the binoculars to enter your eye and its diameter, measured in millimeters, is determined by dividing the aperture by the magnification. Exit pupil diameters will generally vary from 2.5mm to 7.1mm and, as an example, 7x35 binoculars will have an exit pupil of 5mm.
You can see the exit pupil as a circular beam of light in the eyepiece when you hold the binoculars at arm's length. The main importance of the exit pupil is how it relates to the pupil size of your eye. The eye pupil is controlled by the iris, which acts like a variable aperture for the retina and will allow the pupil to change in size from about 2mm up to 8mm, usually depending on the brightness of the available light. When the exit pupil of the binoculars is larger than the eye pupils, some of the light coming from the binoculars will fall on the irises and be undetected by the observer. When the exit pupil is smaller than the eye pupil, then the amount of light falling on the retina will be less than that collected with your normal vision at that particular time, and so the object observed will appear dim. Resolution and contrast are affected adversely, resulting in loss of clarity of the observed image.
On a bright day, an observed image through compact 8x20 binoculars (exit pupil 2.5mm) will appear just as bright as a 7x50s (exit pupil 7.1mm) because the daylight-adapted pupil (2 to 3mm diameter) is the limiting factor in observed brightness under these conditions. At twilight or in other situations of low available light, the eye pupil size will vary between 4 and 5mm. For the best performance under these conditions, a minimum exit pupil of 4mm is essential, such as that provided by an 8x32 or a 10x40 model. At night, when the eye pupil size is at its maximum dilation (6 to 8mm), only binoculars like 7x50s or 10x70s will meet the requirements for the optimum in observed image brightness. So you can see that it's important to consider the exit pupil of binoculars in the context of eye pupil size and the viewing time or conditions of the observer.
There are some other factors to consider when discussing exit pupil. As we grow older, the maximum dilation of our eye pupil diameter slowly decreases in size from about 7 or 8mm at age 20 to about 5mm at age 50. Of course, there will be some individual variation in these figures due to hereditary or environmental factors, but it's worth noting this physiological phenomenon as it relates to exit pupil size and in choosing the binoculars that best meet your needs. Another consideration is whether there will be any observing done under conditions that would not allow you to hold the binoculars steady, such as on the deck of a boat. Here is a situation where a large exit pupil is useful, even for the daylight-adapted eye, because it is much easier to keep the smaller eye pupil centered in the larger exit pupil when the binoculars are difficult to hold steady.
Focusing
There are two basic types of focus systems on binoculars: center and individual focusing. Center focusing is the most common and convenient and generally the most preferred as both eyepieces can be focused simultaneously. Individual focus, which allows for extra-precise focusing adjustments, is often associated with waterproof binoculars, which are used primarily for marine use or for astronomy where the focus distance is seldom changed.
Most observers prefer binoculars that have a slower focus rate which permit very precise, fine focusing. On center focus models, a separate diopter adjustment on one eyepiece (usually the right one) allows compensation for the differences in the strength of each eye.
Phase Correction
By design, roof-prism binoculars split the light entering the barrels into two separate paths. After passing through the objective lens, the light waves are reflected off the surfaces of the roof prism and are split into two out-of-phase beams of light. Light reflected from one roof surface is 1/2 of a wavelength shifted from the light hitting the other roof surface, sometimes referred to as "out of phase" or "phase shift". Although the light waves are subsequently forced back together when they reach the viewer's eye, this phenomenon results in reduced contrast and image resolution. This effect does not occur in Porro prism designs.
A manufacturer can apply a thin coating on the roof prism surfaces of the binoculars that forces the light beams back into phase, thus improving image quality and contrast, creating a sharper view, especially noticeable when viewing fine detail.
