Alignment – Binoculars must not only have their internal lenses and prisms properly collimated, but the two barrels must be parallel so that both are focused on the same spot. If your binoculars are out of alignment vertically and/or horizontally, eyestrain and/or headaches may result. 

– A lens system used in high-end spotting scopes and some binoculars that incorporates multiple glass elements to bring all the colors of the spectrum to the same focus.

BAK-4 barium crown glass
 –  BAK-4 features a higher density and less distortion as light passes through it. Generally considered the superior of the two grades of glass used in most optics, though BK-7 is used effectively in certain applications.

– Essentially two parallel telescopes, binoculars magnify distant objects, making them appear much closer than they actually are. Designed to be hand-held, binoculars are the primary tool of birding.

BK-7 boro-silicate glass
 – Not as dense as BAK-4 glass, it is less expensive and often considered inferior in quality.

Center focus
 – A binocular focus mechanism, most often controlled by a wheel that simultaneously adjusts the images seen in both barrels. A must for birders.

Chromatic aberration
 – Color distortion in an image produced by a lens, caused by the inability of the lens to bring the various colors of light to focus at a single point. Also called (and often detected as) color fringing.

Close focus
 – The near limit at which a binocular or spotting scope can offer a sharp image. For butterfly and dragonfly watchers, something less than 10 feet is needed, and the closer the better. Even for birders, you may need to focus under 10 feet to get a good look at that Mourning Warbler skulking in the goldenrod at Higbee’s Beach.

 – The proper alignment of optics in a spotting scope or binoculars. The lenses and prisms must be factory collimated to focus all wavelengths at a single point. 

Compact or Mini-binoculars
 – Small, lightweight instruments that fit in a pocket, purse, or glove box. They are either small roof prisms with (usually) two hinges or a ‘reversed’ Porro prism, i.e. with the objective lenses closer together than the ocular lenses. Popular among backpackers or for touring but often unsuitable for birding. The cheapest ones rarely offer enough eye relief for eyeglass wearers and often have a narrow field of view. However, the top-shelf manufacturers have excellent options in this category, and Kowa, Opticron, and others now have compact glasses that work for birding.

 – The ability to distinguish differences in brightness between dark and light areas of the image in view. Optics with superior contrast transmits a higher percentage of colors and gives the image a deeper saturation or hue.

Degrees of arc
 – A way of measuring the field of view. Imagine a horizontal circle around you of 360°. If your binoculars have a field of view of 6.7°, picture it as a pie-shaped wedge that gets ever wider as you focus farther away, taking up 6.7° of the 360° around you. A wider field makes getting on a moving target easier.

Depth of field 
– The distance, measured near to far, within which objects seen through a pair of binoculars are in focus. A generous depth of field facilitates viewing birds in woodland; a shallow depth of field makes it difficult to find and hold fast-moving birds. Generally, the depth of field decreases as magnification increases. 

 – The mechanism for adjusting one ocular (usually found on the right barrel) to compensate for any difference (near or far-sightedness) between your eyes.

– Any of the optical components (lenses and prisms) housed in a binocular or spotting scope.

Ergonomic design
 – In binoculars, a shape that follows the contours of the user’s hands and eyes to maximize ease of use.

Exit pupil 
– The circle of light that appears to float above the ocular lens. The diameter, measured in millimeters, can be calculated by dividing the diameter of the objective lens by the power (magnification) of the instrument, e.g. 35 mm ÷ 7x = 5 mm.

In bright daylight, your own pupil may contract to only 2 or 3 millimeters in diameter. When fully adapted to the dark at night, the human pupil dilates to 7 mm. This explains why even compact binoculars such as 8x20 (exit pupil = 2.5 mm) may appear as bright as an 8x40 (exit pupil = 5 mm) on a sunny day—the latter may provide more light than your eye can accept. Conversely, in a high power spotting scope the exit pupil may be as small as 1.2 mm and the image may appear dim even in bright daylight.

The advantage of a large exit pupil at dusk, or in deep woods on a cloudy day now becomes apparent; if your eye’s pupil is at 6 to 7 mm, then an 8x50 with a 6.25 mm exit pupil will give a brighter image of the American Woodcock on the trail at dusk than an 8x25's exit pupil of just 3.125 mm.

One further caveat: as we age, our pupils lose flexibility and the maximum size decreases—to what degree is dependant upon habits such as smoking and genetics. Birders over fifty years old may have a maximum pupil size of 5.5mm, and those over 60 even less. In that case, a 6 mm exit pupil may not functionally be any better for you than one of 5mm. So it may not be worthwhile to lug around a larger objective lens all day long if it won’t be of benefit. You be the judge.

Extra-low Dispersion (ED) glass
 – Fluorite-containing glass in higher-end scope and binocular objective lenses. ED glass focuses the various wavelengths of light on the same plane, creating improved resolution, color and contrast. The purpose is the same as HD or Fluorite or aspherical lenses: to minimize chromatic aberration.

 – The plastic or rubber tube around the ocular lens that helps position the eye the proper distance from the lens, and blocks extraneous side light. These may either twist up and down, or fold. With few exceptions folding eyecups, with only two positions (up or down), are found only on the cheapest optics. Defined click-stops on newer models accommodate the specific eye relief requirements of more users. 

 – The lens that birders peer through when using a spotting scope. Often (but not always) interchangeable, eyepieces may be either fixed power  or variable (zoom) depending on the preferred magnification.

Eye relief 
– The distance, in millimeters, between the ocular lens and the point where the exit pupil is focused. This is especially important for eyeglass wearers, because the outer surface of their eyeglasses may be 8 to 12mm (or more) from their eye. Unless the eye relief provided by the binoculars or scope is greater than this, you may not be able to see the full field of view, and vignetting may occur. As a rule of thumb, eyeglass wearers should look for at least 15 mm of eye relief.

Fast focus
 – A binocular focus system that permits users to move quickly from distant to near objects. Many binoculars can do so in less than two turns of the focus wheel. Avoid binoculars with see-saw type focusing, which is fast, but at the cost of getting or keeping a sharp image without effort.

Field of view
 – The portion of the world around you that you see through a binocular or a spotting scope. Typically described either as the width of the field at a certain distance, e.g. 420 feet (wide) at 1000 yards, or in degrees of arc. To convert degrees of arc to feet at 1000 yards, multiply by 52.5. For example, 6.7° x 52.5 = 351.75 feet at 1000 yards. Field of view is important because the wider it is, the easier to find your target, and the easier to follow a moving one. However, since one way of achieving a wide field might be at the expense of a sharp image, also check the field quality (the part of the field that is in crisp focus). 

Field quality
 – Refers to the resolving quality and consistency of a binocular or scope image as seen across the entire field, either from edge to edge, or from center to edge. Field flattener lenses that give edge-to-edge sharpness aren't cheap.

Focus system 
– The mechanism that controls how optical instruments move the point of focus and maintain a sharp image. An internal focusing system (most recent models) is preferable to an external focusing system.

Fully multi-coated
 – The application of several thin layers of reflection-reducing coatings to all the glass elements of an optical instrument to allow close to ninety-five percent light transmission through the instrument.

Grey market
 - These are items intended by the manufacturer to be sold less expensively in other countries, which are rerouted to the U.S. While it is legal to sell grey market, such items do not come with a USA warranty; you are responsible for any repairs. (Some manufacturers will not provide even non-warranty service for grey market goods.) No authorized dealer sells grey market goods.

High-density (HD) glass
 – See extra-low dispersion (ED) glass.

Image quality
 – What birders actually see when looking through an instrument. Image quality is the product of optical quality and the stability of the image in the glass.

Individual eyepiece adjustment 
– A wheel or knob that changes the focus on just one barrel of the binocular, making it possible to customize optics for the small differences between a binocular user’s eyes. Also called ‘diopter adjustment’.

Interpupillary distance
 – The distance between the centers of your pupils. For the binocular to offer a single, strain-free image, the interpupillary distance of the binocular must be adjustable to match the interpupillary distance of the user’s eyes. Some birders with especially wide or (more often) narrow faces may find that some binoculars just won’t “fit”.

 – The circular piece of glass that transmits and refracts light. The cross-section varies depending on use; it may be double-convex () , plano-convex |) , plano-concave |( , double-concave )( , or aspherical. A high-end roof prism binocular may have as many as eight or nine lens elements in each barrel.

Lens coatings
 – Thin layers of magnesium fluoride or other chemicals that are baked onto the surfaces of lenses and prisms to prevent light from being reflected away as light strikes the glass.

 – Also called ‘power’. The degree to which an object is enlarged when looking through a binocular or spotting scope. It is the first number in a binocular designation and paired with the multiplication symbol, e.g. 8x 42, which means it enlarges eight times or makes the object appear eight times closer. For you real optics buffs, power is determined by the focal length of the objective lens divided by the focal length of the ocular. For example, 160mm ÷ 20mm = 8x.

Marine binoculars
 – Binoculars whose primary function is aboard ship and whose primary need is the ability to withstand frequent dousing with water. In order to assure that ability, the eyepieces focus individually. Typically large and heavy to provide a bright view between dusk and dawn and during storms.

Multilayer coatings 
– The application of several very thin layers of reflection-reducing coatings to the glass elements of an optical instrument.

Nitrogen Purged
 – Refers to the process when the atmospheric air within the binocular is replaced with nitrogen. This prevents mold, mildew or acid etching of binocular lenses and prisms that atmospheric air (containing moisture) would permit. Most waterproof binoculars are nitrogen purged. Some companies use Argon to the same effect.

Objective lens
 – The “big end” of the binocular or scope, which gathers the incoming light - the diameter of this lens in millimeters is the second number in a binoculars description, e.g. 8 x42(mm). As objective size increases it gathers more light, and the theoretical resolution also increases.

Ocular lens
 – The lens you put your eye to, where the exit pupil is found; the “small end.” This lens magnifies the image produced by the objective lens.

Phase coating (
also called phase corrective coating) – A chemical coating applied to the elements of roof prism binoculars to correct a light-wave shift that is inherent in roof prism design.

Porro prism
 – Named for Italian inventor Ignazio Porro, this is the “classic” binocular design type—wide bodied because the objectives lenses are typically farther apart than the ocular lenses—that offers superior light transmission (and some feel a more three-dimensional image) at a lower cost than the roof prism design. Because these tend to make you hold your elbows farther apart, one tends to get either a less-stable image or faster arm fatigue with Porro prism compared to roof prism designs.

 – See Magnification.

 – One of the glass elements found in binoculars and most spotting scopes. Their primary function is to erect and flip the images produced by the objective lenses so they appear right-side-up, and correct in length from left to right. An added benefit is that by “folding” the light path the optical instrument can be shortened considerably.

Range of focus
 – The number of revolutions required of a focus wheel to go from close focus to “infinity”. In binoculars, the ideal range is one full turn, though some appreciate a less critical or “touchy” focus of 1¼ to 1½ turns.

Reflecting scope
 – A spotting scope that uses a concave parabolic mirror as the primary light-gathering element. Reflecting scopes are popular among astronomers but less so among birders. May require frequent collimation. 

Refracting scope
 – A type of spotting scope that uses a large lens as the primary light-gathering element. Slimmer and generally more rugged than reflecting scopes, refractors are the preferred scope among birders.

Relative brightness
 – A measure of an instrument’s brightness determined by squaring the diameter of the exit pupil. As a mathematical function it nominally doesn’t vary among manufacturers. For instance, all binoculars with a 5mm exit pupil have a relative brightness of 5 x 5 = 25. This doesn't take into account varying rates in light transmission of different glass and coating types.

 – The capacity of a binocular or scope to discern detail. Astronomers describe it in terms of seconds of arc (60 minutes in 1° and 60 seconds in 1 minute, hence 3600 arc seconds in 1°). Digital photographers talk about mega pixels. For birders, it’s about whether you can see the subtle marking on the tertials of a juvenile Short-billed Dowitcher—or not.

Roof prism
 – Today this is the most popular binocular design, with a narrow chassis offering greater stability. The light-gathering system reaches parity with the porro prism design but at greater cost.

Spotting scope
 – A single-barreled, optical aid whose advantage is higher magnification and whose primary function is the study of very distant birds or the detailed study of birds that are difficult to separate from other species.

Stabilized binoculars 
– A binocular with an internal mechanism that reduces vibrations caused by hand shake. While they do work, such optics usually come with downsides such as greater weight and a smaller range of interpupillary adjustment.  They often carry a hefty price tag (up to several thousand), sometimes in the range of high-end optics – but without the high-end optics. Opticron just introduced two smaller models at under $800.

– Refers to distracting black crescents that optics users see most often when the interpupillary distance or eye-relief is not properly adjusted.

Zoom binoculars/scopes 
– An instrument whose magnification can be increased or decreased by moving a lever. In binoculars, this feature usually compromises performance and is commonly regarded as an inferior product. However, in many high-quality spotting scopes a zoom eyepiece can be as optically precise as a good quality camera zoom lens, and very useful.