Sport Optics Glossary of Terms
February 2, 2021
Aberration – defects in images produced by an optical system caused by limitations in the design and manufacture of the optics. For more information, click here.
- Astigmatism – an off-axis effect that causes rays of light to come to different focal points in the vertical and horizontal plane. For more information, click here.
- Coma – an off-axis aberration that occurs when the rays from successive zones are displaced outward relative to the principal ray. For more information, click here.
- Chromatic Aberration – the inability of a lens to focus the various wavelengths of light to a single point. Chromatic aberration decreases contrast and makes it difficult to obtain a sharp image. For more information, click here.
- Field curvature – a defect that occurs when a large object or multiple objects scattered in a field of view do not focus on a flat surface. Objects at the center of the field will be in focus, but those at the edges won’t be in focus. For more information, click here.
- Spherical Aberration – the failure of rays passing at different distances from the center of a lens or mirror to come to the same focus. For more information, click here.
- Achromat or Achromatic Lens – a system of two or more lenses that is substantially free from chromatic aberration and in which the lenses are made of different substances so that the focal length of the system is the same for two or three wavelengths of light.
- Aperture – reference Objective Lens.
- Balance plate – a plate found on spotting scopes to properly balance the scope when attached to a tripod.
- Barlow Lens – a diverging lens which, when used with an eyepiece, can double or triple a telescope’s focal length.
- Binocular – an optical instrument for viewing distant objects with two eyes.
- Close Focus – the closest distance to an object that the binocular/spotting scope will focus sharply. For more information, click here.
- Collimation – the alignment between the two optical tubes of the binocular. For more information, click here.
- Color Fringing – reference Chromatic Aberration.
- Contrast – the degree to which both dim and bright objects in the image can be differentiated from each other and from the background of the image. For more information, click here.
- Depth of Field – the distance in front of and beyond the subject that appears to be in focus.
- Dew shield – an accessory that protects your optics from excess moisture forming on the corrector lens and helps block out stray light for better viewing and astroimaging.
- Dielectric Coating – reference prism coating.
- Digiscoping – attaching your camera or smartphone to your optics to captures photos or video. For more information, click here.
- Diopter Adjustment – turning of the ring located below/on the eyepiece itself that allows you to compensate for the difference in eyesight between your right eye and left eye. Adjusting the diopter offsets the lenses in the two barrels so the image is sharp for each eye. For more information, click here.
Distortion – a visual defect that occurs when the image viewed through an optic differs geometrically from the object being viewed.
- Barrel distortion – a curving outward of the lines as you look near the edges of the field.
- Pincushion distortion – a curving inward of the lines as you look near the edges of the field.
- ED Glass – optical glass with extra low dispersion, which reduces the visual defect known as chromatic aberration. For more information, click here.
- Exit Pupil – the width of the beam of light leaving the eyepiece, usually measured in millimeters. For more information, click here.
- Eye Relief – the distance in millimeters between your eye and the binocular/spotting scope eyepiece while observing the full field of view. For more information, click here.
- Eyecup – shades that prevent extraneous light from distracting your vision. Eyecups can come in a flexible rubber material or hard material. For more information, click here.
Eyepiece – an ocular lens that magnifies objects and is an essential part of visual observing. For more information, click here.
- Plössl Eyepiece – a symmetrical eyepiece. Uses a 4-lens design: two-group, two-element design that uses back-to-back plano-convex front element and eye lenses. Apparent field of view is 50◦ or more.
- Prime Eyepiece – an eyepiece designed to yield one specific level of magnification respective to an optical instrument’s focal length.
- Zoom Eyepiece – an eyepiece with the ability to rotate the barrel to increase magnification.
- Field Curvature – a visual defect that occurs when lenses do not focus images on a plane but on a curved surface. When images suffer from field curvature, objects in the center of the field of view are in focus, but the edges may not be in focus.
- Field Flattener – a type of lens in binoculars that is placed at the focal plane of your optics that counteracts concave or convex field curvature, providing an image closer to being in focus over the full field.
Field of View (FOV) – the width of area in feet/meters/yards that can be seen at 1,000 yards/meters. For more information, click here.
- Angular Field of View (AOV) – the true angle seen through the optics and is usually measured in degrees. For more information, click here.
- Linear Field of View – width of the area seen and is given in feet observed at 1000 yds
- Flatness of Field – a measure of edge-to-edge sharpness of the viewed image.
- Focal Length – the distance from the objective lens to the focal plane (the place where the image forms).
- Focus Wheel – an adjustment knob in the center or on an eyepiece of your binocular that lets you adjust focus based on the distance of your object.
- Fogproof – reference nitrogen-filled.
- HD Glass – reference ED Glass.
- Interpupillary distance (IPD) – the measurement or the potential range of adjustment available between a binoculars' exit pupils, as measured in millimeters. For more information, click here.
- Lens Coating – reference Optical Coating.
- Lens Shade – a tube extension of up to several inches that provides shade from distracting light and, in cool temperatures, prevents dew from forming on the objective lens. For more information, click here.
- Light pollution – unwanted light that emitted into the night sky by manmade sources (city lights, flashlights) that significantly diminishes the views of faint objects in the sky. For more information, click here.
Magnification – the amount that an optic appears to bring the subject closer to the observer. The magnification of the instrument is the first number listed in a binocular’s name (i.e., an 8x42 binocular has 8x magnification). An 8x binocular appears to bring the subject 8 times closer to the observer than viewing with the unaided eye. For more information, click here.
- High Magnification (10x+) – appears to bring the subject closer but could provide lower light transmission and a narrower field of view than low magnification optics. For more information, click here.
- Low Magnification (<10x) – provides less viewing “power” or magnification, but generally delivers increased light transmission and wider field of view than high powered optics. For more information, click here.
- Maksutov-Cassegrain – a catadioptric spotting scope that uses a spherical curved corrector plate to reduce off-axis aberrations such as coma found in reflecting telescopes while avoiding chromatic aberration. For more information, click here.
- Monocular – a handheld optical instrument for viewing distant objects with one eye.
- Monopod – a one-legged support for a binocular or spotting scope. For more information, click here.
- Nitrogen-filled – the optic has been filled with dry, inert gas (nitrogen) as an additional barrier to the buildup of internal moisture to prevent internal fogging.
- Objective Lens – the lens that faces towards the object being viewed. The diameter of the instrument’s objective lens is rendered in millimeters and is the second number listed in a binocular’s name (i.e., an 8x42 binocular has a 42mm objective lens). For more information, click here.
- Ocular Lens – the outermost lens in the instrument’s eyepiece. You look into the ocular lens to view through the instrument.
Optical Coating – one or multiple layers of coating strategically deposited on mirrors and lenses to reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast. For more information, click here.
- Coated optics – one or more surfaces have optical coatings applied.
- Fully coated – all air-to-glass surfaces have optical coatings applied.
- Multi-coated – one or more surfaces have multiple layers of optical coatings applied.
- Fully multi-coated – all air-to-glass surfaces have multiple layers of optical coatings applied.
- Optical Glass – specialized manufactured glass that is comprised of chemical composition, melting process, and finishing methods. Examples are ED Glass and Prism Glass.
- Power – reference Magnification.
Prism – a transparent glass optical element with flat, polished surfaces that refract light. For more information, click here.
- Porro Prism – a reflective optical prism in the form of a right-angle triangle with two sides of equal length. The Porro prism optical system creates an enhanced 3D view of objects due to its wider spaced objective lenses. For more information, click here.
- Roof Prism – a reflective optical prism containing a section where two faces meet at a 90° angle. The Roof prism simulates the image you naturally see but does not provide an enhanced 3D view of the object. For more information, click here.
Prism Glass Types
- BaK4 – light barium crown glass (baritleichtkron in German). This glass has a higher refractive index and provides better quality images. It is very common is mid to high-end price ranges. For more information, click here.
- BK7 – a type of borosilicate glass that has long been and still is widely used across many different optical product brands and models. For more information, click here.
- K9 – reference BK7.
Prism Coating – a multilayer coating to raise transmittance. For more information, click here.
- Dielectric Coating – a high-reflectivity prism coating that provides high reflectivity across the entire range of visible light and acts as a dielectric mirror, providing both sharper colors and crisper images. For more information, click here.
- High Reflectivity Metallic Coating – a metallic material such as aluminum or silver that is applied to the reverse side of a prism surface that is not totally reflective to raise the reflectivity of the prism mirror surface. For more information, click here.
- Phase Coating – a type of optical coating that is applied to one surface of the shorter light path half of a roof prism to ensure that the image emerging from it remains "in phase." With phase-corrected prisms, no colors are reinforced or cancelled, thus providing more accurate color reproduction. For more information, click here.
- Silver Alloy – a high-reflectivity prism coating that acts as a mirror, providing both sharper colors and crisper images when compared to aluminum prism coatings. A silver alloy typically has a reflectivity of 95-98%. For more information, click here.
- Refractor – an optical design with a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. This is the most popular optical design for spotting scopes. For more information, click here.
- Relative Brightness – a number used to compare the brightness of binoculars or spotting scopes of similar magnification. The larger the relative brightness number, the brighter the image. For more information, click here.
- Resolution – the measure of the ability of a spotting scope or binocular to distinguish fine details.
- Reticle – a measuring device, like crosshairs, placed in an optical system to estimate range of an object by finding a known dimension on either the target or on an object that you believe is near the target. For more information, click here.
- Rotating Tripod Mount Ring – a ring found on spotting scopes that allows you to change the orientation of the eyepiece to place it in the best viewing position.
- Schmidt-Cassegrain (SCT) – an optical design that features combination of mirrors and lenses (catadioptric optics) to fold the optics and form an image. The light enters through a thin aspheric Schmidt correcting lens, strikes the spherical primary mirror, reflects the image on a small convex spherical secondary mirror, and then reflects the light to the rear of the instrument. For more information, click here.
- Sighting Line – a marking on a spotting scope’s lens shade that aids in locating your subject and bringing it into the field of view.
- Solar Filter – specially designed filters that fit over the objective end of the optic and block most of the Sun’s to avoid permanent, irreversible eye damage. You should never point your optic at the Sun without a proper solar filter. For more information, click here.
- Sport Optics – a general term that describes binoculars, spotting scopes, and monoculars.
- Spotting Scope – a small, portable telescope with added optics to present an erect (correctly oriented) image, optimized for the observation of terrestrial objects.
- T-Adapter – an accessory used with a T-Ring that turns a spotting scope or telescope into a camera lens for your SLR or DLSR camera. T-Adapters can be universal or optical tube specific.
- T-Ring – an accessory used with a T-Adapter that attaches your SLR or DSLR camera to a spotting scope or telescope for terrestrial or astrophotography. T-Rings are camera brand-specific.
- Tripod – a three-legged support for a binocular or spotting scope. For more information, click here.
- Tripod Adapter – an adapter that lifts the weight of binoculars off your arms and onto the steady support of a tripod.
- Twilight Factor – a number used to compare the effectiveness of binoculars or spotting scopes when used in low light. For more information, click here.
- Vignetting – a situation in an optical system in which the field of view is not fully illuminated at the edges.
- Waterproof – equipment that is sealed and filled with a dry, inert gas (usually nitrogen) that contains no water vapor to condense inside. For more info, click here.
- Water Resistant – equipment that is sealed to withstand splashing water or moderate rain but may eventually leak if the bad weather is prolonged or extreme. For more info, click here.
- Zoom – an optic with variable magnification levels that can be adjusted with a zoom lever to increase or decrease magnification.