2/28/2013

Every area of interest, particularly if it’s the least bit technical, has its own specialized vocabulary. For example, when the Global Positioning System (GPS) came along and I decided to get a receiver for field use, I had to learn the precise meanings of a number of esoteric terms and abbreviations, which often have different meanings than in our everyday world. For example, an “active leg” isn’t something that twitches while you sleep and a “last fix” has nothing to do with illicit drug use. Specialized technical terms serve the purpose of compressing very complex ideas into one or two words. Consequently, one must either learn the jargon or remain ignorant to what is being discussed. Likewise, one cannot fully understand the workings of sports optics (binoculars, spotting scopes and telescopic sights) without knowing about diopters, erecting systems, exit pupils, parallax and so forth.

Aberrations: Optical abnormalities present in lens systems, which, though correctable, cannot be completely eliminated. They include: chromatic aberration, spherical aberration, coma, distortion, curvature of field and astigmatism.

Achromat: Also known as an achromatic lens. A lens consisting of two or more elements, usually of crown and flint glass, that has been corrected for chromatic (color) aberrations with respect to two selected colors, usually blue and red.

Angular Measurements: The performance characteristics of optical instruments are often expressed as angular measurements. Field of view is measured in degrees; windage and elevation adjustments are calibrated in minutes of angle or fractions thereof; resolution is measured in seconds of angle; and the center-to-center spacing of dots on mil-dot reticles equals one milliradian (mil.).

Degree of Angle: A unit of angular measure equal to 1/360 of a circle. One degree subtends 62.83 inches at 100 yards or 52.36 feet at 1,000 yards.

Minute of Angle (m.o.a.): A unit of angular measure equal to 1/60 of a degree, or 60 seconds. One minute subtends almost exactly 1 inch (1.047") at 100 yards, 2 inches at 200 yards, 3 inches at 300 yards, and so forth—making it a criterion for firearm accuracy.

Second of Angle (s.o.a.): A unit of angular measure equal to 1/60 of a minute or 1/3600 of a degree, which subtends 0.01745 inches at 100 yards. Resolving power is usually stated in seconds of angle.

Mil: A unit of angular measure that is subtended by one unit of length at a distance of 1,000 units of length, such as one meter at a distance of 1,000 meters. The spaces between dot centers on mil dot reticles subtend 3.6 inches at 100 yards, 7.2 inches at 200 yards, 10.8 inches at 300 yards and so forth.

Apochromat: Also known as an apochromatic lens. A lens system in which the three primary colors (blue, green and red) have been brought to a common focus through the use of special optical glasses, thus eliminating practically all chromatic aberration. Abbreviated terms used to designate instruments with apochromatic optics include: ED (Extra-low Dispersion), HD (High Definition), FL (Fluorite) and APO (Apochromatic).

Binocular: adj. Relating to, used by, or involving both eyes at the same time, as in binocular vision. n. A pair of telescopes mounted side-by-side, usually by means of a centrally located hinge, so that the eyepiece spacing can be adjusted to match the viewer’s interocular distance. Binoculars are rated by their magnifying power followed by the objective diameter in millimeters (e.g., 8X 42 mm). The basic types of binoculars include:

•    Galilean Binocular (opera glasses): Lacking prisms, these small binoculars—consisting of convex objective lenses and concave ocular lenses—are generally unsuited for hunting or other long-range viewing because of their narrow fields of view and low magnifying powers, which range from about 2.5X to 4X.

•    Porro Prism Binocular: This classic binocular design has a Z-type configuration that positions the objective lenses farther apart than the eyepieces, significantly increasing the viewer’s stereoscopic depth perception.

•    Reversed Porro Binocular: Though similar to conventional Porro prism models, this binocular has the prisms rotated so that the objectives are closer together than the eyepieces. This is characteristic of compact binoculars, although some full-sized binoculars now position the objectives as close together as possible. In either case the stereopsis is decreased.

•    Roof Prism Binocular: This type is recognizable by having its objective and eyepiece lenses in a straight line. The stereopsis is then equal to that of the viewer’s unaided eyes as enhanced by the magnification. Although, some roof prism binoculars, particularly those having Abbe-style roof prisms, do have the objectives slightly offset.

Center Focus: Refers to a type of binocular with a centrally located focusing control that adjusts both eyepieces of a binocular simultaneously. Such binoculars usually have an individual diopter adjustment for one eyepiece to compensate for vision differences between the viewer’s two eyes.

Coated Optics: Optical elements having refractive or reflective surfaces that have been coated with one or more layers of either dielectric or metallic materials that serve to reduce or increase reflections, eliminate phase interference, decrease external fogging or protect the surfaces from abrasion. Magnesium fluoride, silicon oxide, zinc sulfide, aluminum and silver are a few of the most commonly used coating materials. The primary coatings include:

•    Antireflection Coatings: These coatings, whether single layer or multilayered, reduce reflections from optical surfaces, thus increasing the percentage of the available light transmitted through optical instruments. The results are improved image brightness and contrast.

•    Reflection Coatings: These coatings improve the reflectance of the various types of mirrored surfaces used in optical instruments. Any light lost in the process of reflection reduces an instrument’s image brightness.

Phase-Correction (P) Coatings: When applied to the roof surfaces of roof prisms, phase-correction coatings reduce the image-degrading affects of destructive interference that occur when out-of-phase light waves from the opposite roof surfaces reunite to form images.

Collimation: The process of aligning the optical axes of optical systems to coincide with the mechanical axes of surfaces of an instrument. Binoculars pose a special problem because both optical systems must be pointed in precisely the same direction; otherwise the viewer will experience eyestrain or worse, double imaging.

Collimator: An optical device consisting of a well-corrected objective lens with a reticle at its focal plane. An example being the lens systems used in optical bore sighters.

Diopter: A measurement unit that expresses the refractive power of a lens. A lens with a focal length of 1 meter has a refractive power of one diopter. Lenses with shorter focal lengths have greater power in diopters. Accordingly, a lens with a focal length of one-quarter meter has a power of four diopters. The refractive power of converging (convex) lenses is positive; the refractive power of a diverging (concave) lens is negative.

Diopter Scale: A focusing scale—such as on the eyepieces of binoculars—which serves to indicate the degree of convergence or divergence of the light rays emerging from the instrument.

Erecting System: Lenses or prisms that serve to erect the inverted (upside down and laterally reversed) image formed by objective lenses. All telescopic sights and many telescopes use lenses to erect the image, while binoculars and prismatic spotting scopes use either Porro or roof prisms. Astronomical telescopes don’t need and, therefore, seldom have erecting systems.