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Glossary For Optics

Free Stuff May 22, 2008

Optical Glossary

Achromatic
A lens classed as achromatic has been formulated to be free of chromatic aberration, which causes colour fringing. An achromatic lens, or achromat, achieves identical focus with light rays of two different colours.

Apochromatic
Apochromatic lenses are corrected to bring all three primary colours to the same focus.

Astigmatism
Aberration in a lens or optical system that makes lines oriented in certain directions less sharply focussed than lines running in other directions. Astigmatism also occurs in the human eye. A typical astigmatic effect would be to see the vertical lines on a sheet of graph paper sharply while seeing the horizontal lines less clearly, or vice versa.

Bell
In scope sights with ends flared to accommodate large-diameter ocular or objective lenses, the oversized lens housings are referred to as bells, as in ocular bell or objective bell.

Binocular Collimation
Regulating the two halves of a binocular for parallelism. If the halves are misaligned, the images they form will not merge into a single, sharply defined viewing image. Severe misalignment causes a perceptible multiple-image effect. Slight misalignment may not be specifically obvious, but will result in eyestrain during prolonged viewing.

Catadioptric Lens
A lens system containing front-surface mirrors in addition to refractive lenses. Catadioptric designs “fold” the light path internally to reduce the overall length of the lens system to a significantly shorter dimension than a conventional design would require for a given focal length. Catadioptric lenses, also called mirror lenses, are sometimes used as compact telephoto camera lenses and are occasionally employed in spotting scopes. They are relatively common in astronomical telescopes.

Centre Focus
Refers to a type of binocular with a central focussing control that adjusts both halves of the binocular simultaneously. In centre-focus binoculars, one ocular lens is individually adjustable for focus while the other is fixed. Initially, the user focuses the fixed-ocular half of the instrument with the central control, then focuses the other half with its independently adjustable ocular. Further focussing requires using only the centre control. Centre focus binoculars are quick and easy to use, but are relatively complex mechanically.

Chromatic Aberration
Occurs when a lens brings different wavelengths (colours) of light to separate and distinct points of focus rather than to a common focus. Depending upon the severity, the effect may range from very slight unsharpness to gross unsharpness with visible colour fringing.

Collimator
An optical device containing a lens system that bends light rays passing through it to make the rays parallel. The lens system is often coupled with a target such as a reticle or, in certain applications, a pinpoint light source. A collimator is used to “trick” another optical device, such as a camera lens, telescope or scope sight, into forming an image of the collimator’s reference target as though it were a distant target rather than a very close one. In the optical industry, collimators are widely used for visually assessing the image-forming characteristics of lenses or lens systems. In the context of sporting firearms, small collimating units, also called boresighters, can be used to regulate a scope sight’s reticle to approximately match a shoulder arm’s bore axis in the relative comfort of shop or home, without making a trip to a firing range. The scope “zero” achieved by careful use of a collimator should be considered simply a rough setting that must be refined by actual test firing.

Coma
Off-axis lens aberration that distorts oblique light rays transmitted by the lens. Coma produces taillike smears on tiny round image details such as points of light. Under high magnification, coma may make tiny points of light resemble miniature comets.

Crosshairs
A cruciform reticle in an optical sight, also commonly referred to as crosswire’s.

Curvature of Field
Off-axis lens aberration that makes light rays come to a focus at different points along a curved, rather than flat, focal plane. When viewing a flat subject, such as a paper target or a brick wall that is perpendicular to the lens axis, curvature of field will make it impossible to obtain sharp focus at the centre and edges of the viewing field simultaneously. When the centre is sharp, the edges will be soft; altering focus to sharpen the edges of the image will blur the centre.

Diaphragm
A mechanical device for reducing the amount of light entering or transmitted by an optical system. In simplest, non-adjustable form, often called a stop or Waterhouse stop, the diaphragm is nothing more than an opaque plate configured to fit in a slot or holder in the lens system, or to attach over a lens. The plate has a hole of the desired size, usually circular, that allows a limited amount of light through the optics. An adjustable diaphragm, such as found in most camera lenses and called an iris diaphragm, consists of several interconnected, overlapping movable metal or composition blades that can be opened and closed concentrically to vary the size of the central aperture. Using a stop or diaphragm to reduce the effective aperture of a viewing system produces two immediately evident effects. First, the brightness is reduced. Second, the depth of field is increased. Depth of field refers to the zone of sharpness from near to far in the observed image. Accessory stops or diaphragms available for some target-model rifle scopes are used to enhance sharpness in depth, which is quite limited with high-magnification optics. Iris diaphragms or interchangeable stops are often used in conjunction with high-precision aperture sights on match rifles to achieve optimum clarity of front sight and target. Handgun shooters who cannot see iron sights clearly without corrective spectacles that render the target unacceptably blurry may manage to sharpen sights and target adequately by placing an accessory small-aperture diaphragm or stop plate over the aiming eye’s eyeglass lens.

Diopter
A measurement unit that expresses the refractive power of a lens. Focussing eyepieces of scope sights and binoculars are sometimes equipped with a scale calibrated in diopters to facilitate returning to a previously determined setting. The corrective power of simple eyeglass lenses is given in diopters. Accessory diopter correction lenses are sometimes available for installation in viewing systems or sights to allow comfortable use while wearing non-prescription shooting glasses.

Distortion
A distorted image falsifies subject contours and/or proportions. Distortion is most easily perceived when viewing subjects of known shape and dimensional relationships. Two common forms of distortion are barrel distortion and pincushion distortion. Barrel distortion makes straight lines appear to bow outward toward the edges of the field. Pincushion distortion makes straight lines appear to bow inward toward the centre of the field. Small amounts of distortion may be tolerable in viewing optics provided the distortion remains fairly unobtrusive, as it generally does not impair overall utility. Nonetheless, the finest viewing optics are virtually free of distortion.

Erector lens
Certain combinations of objective and ocular lenses yield an inverted image. An erector lens incorporated into the system serves to reorient the image right side up. In binoculars and telescopes, prisms are often used to “erect” the image.

Exit Pupil
The exit pupil of an optical viewing system appears as a bright disc of light that may be seen by looking into the ocular lens from about 10″ with the instrument aimed at a light source (not the sun!) or a bright background. Within certain limits, the larger the exit pupil, the brighter the view through the optical system. You can calculate the aperture of the exit pupil in millimetres by dividing the effective diameter of the objective lens in millimetres by the magnification of the instrument. An exit pupil aperture as small as 2 mm is large enough for easy viewing in good daylight conditions. An exit pupil diameter of approximately 7 mm is large enough for darker environments or evening use. As the pupil of a normal human eye cannot open larger than about 7 mm, there is little or no practical brightness advantage to designing ordinary viewing instruments with excessively large exit pupils.

Eyepiece
The lens or lens group through which the user of an optical instrument views the image. The term eyepiece is often used interchangeably with “ocular” or “ocular lens,” but may also refer to the physical rim or mount flange that contains the outermost lens of the ocular. The context in which the word occurs normally reveals the meaning.

Eye Relief
The distance between the user’s eye and the ocular lens at which the entire field of view of a scope sight or other optical device is visible simultaneously. Instruments described as having long or extended eye relief allow more than the usual distance between eye and ocular. Long-eye-relief rifle scopes are desirable on rifles that recoil strongly to reduce the likelihood of the ocular striking the shooter’s eye or face. Handgun scopes with extended eye relief provide a full field of view when held at arm’s length. Long-eye-relief spotting scopes and binoculars allow users to see the entire field of view while wearing eyeglasses.

Field of View
The expanse of subject included within the field of an optical system. The measurement is normally taken across the diameter of a circular field and, in the US, expressed, for scope sights, in feet or yards at a subject distance of 100 yards unless otherwise stated. The field of view of binoculars is generally given for 100 or 1000 yards. In countries using the metric system, field of view is commonly expressed in metres at a subject distance of 100 or 1000 metres. Field of view may also be specified in angular terms, as an angle of view. Knowing the angle of view allows calculating the field of view for any subject distance of interest.

Flare
The scattering of non-image-forming light within an optical system, caused by poorly controlled internal reflections from lens surfaces or mechanical components. Flare may be perceived as stray reflections, as a veiling glare or hazy quality that obscures detail in the image, as a loss in image contrast, or as any combination of these phenomena. It is most likely to occur when the field of view includes highly reflective, brightly lighted features or a strong light source. Some target scopes and spotting scopes are equipped with lens shades that may be quite effective in suppressing flare by preventing sunlight from shining directly on the front element of the objective lens in side-lighting or when shooting against the light. The higher the quality of the optical system, the less likely it will be to exhibit flare effects, although no optical system is immune to flare under extreme’ lighting conditions.

Focal Plane
The plane where the image formed by the lens or lens system is in sharp focus. In a camera, the focal plane is the sensitized surface of the film.

Focus
This term may refer to the focal point of a lens, a lens’ focal length, the condition of maximum clarity of an image or the action of adjusting a lens system to yield the clearest and best defined image it is capable of forming of a subject at a particular distance. An unfocused or out-of-focus image appears blurry, ill-defined and deficient in contrast. An in-focus image looks sharp, precisely defined and as rich in contrast as the lens system and viewing conditions permit.

Focussing Scale
Scope sights with adjustable focussing objectives usually have a focussing scale on or adjacent to the movable focus control. The scale, calibrated in feet,yards or metres, permits setting the focus rapidly for targets at known distances. When target distance is unknown, the scope is focussed visually and the scale ignored. Accuracy of focussing scale calibrations should be verified when a scope is new and occasionally thereafter, as discrepancies are not unusual. This may be done by focussing the scope carefully on targets at known distances. If the focussing scale is at odds with reality, have it adjusted by the appropriate service facility or remark it with small pieces of tape or dots of nail polish for personally useful range settings. An optical instrument that focuses erratically or inconsistently should be repaired or replaced.

Haze
Light scattered by particulate matter in the atmosphere, such as dust or moisture droplets. Haze lends a foggy or cloudy appearance to distant objects or scenes, subduing colours and contrast. Haze effects are more apparent when using high magnification optical instruments than when viewing with lower-power optics or the naked eye, and are more pronounced at long range than short range under a given set of atmospheric conditions.

Individual Focus
Refers to a type of binocular in which the ocular of each half must be focussed independently each time the instrument is refocused. Adjusting the focus of an individual-focus binocular is slower than a centre-focus model, but the former is mechanically simpler and, construction quality being equal, potentially less vulnerable to hard knocks.

Lens Aberration
An optical flaw that adversely affects image quality.

Lens Coating
Anti-reflection layer or layers deposited on surfaces of prisms and lenses by vaporizing metallic compounds in vacuum chambers containing the optical elements to be coated. Lens coating reduces loss of light and image degradation caused by scattering of light reflected by lens surfaces. Traditional lens coatings comprise a single anti-reflection layer applied to each significant glass surface. More modern multiple-layer lens coatings consist of three or more anti-reflection layers per surface. Both single and mufti-layer coatings are effective when they are applied appropriately.

Luminosity
Radiating or reflecting light. Also a measure of the brightness of alight source or of a reflective surface expressed quantitatively according to rigorously defined criteria.

Magnification
Relationship between the apparent size of an object as viewed through an optical instrument and the apparent size of the object as seen with the naked eye. For example, a scope that makes an object look four times larger than it appears when viewed directly has a magnification of four times, usually expressed as 4X. A magnification designated by a number smaller than one indicates that the viewing device makes the subject appear proportionately smaller than when seen with the unaided eye. An instrument of 0.9X magnification reduces the apparent size of the subject to only 90% of what it would seem in direct viewing. Note that the term magnification is used even when the optical device “minifies.” As a practical matter, nearly all sporting optics yield positive magnifications that enlarge the viewing image. The magnification bears a direct relationship to the apparent distance of the viewed object: the greater the magnification, the closer the object appears. Dividing the actual distance of the object by the magnification indicates the apparent distance of the object as viewed through the optical system. An object 100 yds. distant would appear as though it were 50 yds. distant when viewed through a 2X scope and just 10 yds. distant through a IO X scope.

Mirage
Optical phenomenon that occurs when air near the ground is significantly denser than the air above it, creating visible reflected images of distant objects or targets. Less extreme mirage phenomena are of concern to outdoor target shooters, appearing as a shimmer that makes precise aiming difficult or impossible, or that may cause an apparent displacement of the target that leads to erroneous shot placement as the shooter fires at the mirage image rather than the actual target. High-power scope sights make mirage more noticeable, but they also allow the experienced marksman to judge the mirage effect with greater certainty, and compensate for it to the best of his or her ability. The term mirage is also broadly applied to the heat shimmer from a hot gun barrel, which may disrupt the sighting image. Extra-long lens shades offer relief from such heat shimmer.

Objective
The objective lens or lens group forms the image of a distant target or subject at or near the focal point of the optical system. The objective is located at the front of the instrument.

Ocular
The ocular lens or lens group enlarges the image the objective lens forms and allows the viewer’s eye to see it clearly despite its proximity to the eye. The ocular is the part of the optical system nearest the viewer’s eye and is the rearmost lens group of a scope sight. The ocular lenses of nearly all rifle and handgun scopes may be focussed to suit the user’s vision, thus providing the sharpest possible view of the reticle and of the image formed by the scope’s objective lens.

Parallax
Apparent shift in position of a viewed object attributable to the difference between two separate and distinct points of view. In a scope sight, parallax can cause an aiming error, or parallax error, when the target image is not formed in the same plane as the reticle. The condition may be detected by moving the aiming eye progressively away from the centre of the ocular toward the edge of the lens without moving the scope. If the target and reticle shift position slightly relative to each other, parallax error exists and will cause a corresponding shift in the centre of impact. The more the eye moves away from the scope’s optical axis, the greater the parallax error. Parallax error does not occur when the aiming eye is well centred with respect to the ocular lens, even though the conditions for a potential error are present. With a fixed-focus scope sight that has been factory-set for optimum focus at a specific distance, the potential for parallax error exists whenever targets are nearer or farther than the range for which the optics were regulated. In most cases the amount of error will not be significant in the context of normal field shooting. Parallax error can be avoided satisfactorily by keeping the aiming eye reasonably well centred. High-power varmint and target scope sights have adjustable objectives that permit focussing the scope over a wide range of target distances. Focussing such scopes carefully assures maximum image sharpness and also eliminates potential parallax error at the distance for which the scope is focussed. With the scope focussed properly, moving the eye off centre will not cause a shift in the relative positions of reticle and target, and the centre of impact will not be affected by the shift in eye position relative to the optical axis.

Objective lens
Used in pairs in a binocular barrel between the objective and ocular lenses, right-angle porno prisms erect the inverted image formed by the objective lens group. They also “fold” the light path, permitting the binocular barrel to be made shorter than if a non-prismatic optical design were used. The offset required between the porno prisms in each pair gives the binocular barrels a corresponding offset between the objective and ocular sections.

Relative Brightness
Relative brightness numbers are an attempt to quantify the “brightness” of scope sights and binoculars to facilitate comparison. The relative brightness number is the square of the diameter of the instrument’s exit pupil, expressed in millimetres. You can determine the size of the exit pupil, if it isn’t stated in product literature, by dividing the useful aperture of the objective lens, in millimetres, by the magnification, or power, of the instrument. For example, a 4X rifle scope with a 40 mm objective would have a 10 mm exit pupil (40 = 4 = 10) and a relative brightness of 100 (l Oz). The relative brightness reflects the optical truth that, all other factors being equal, large-diameter objectives admit more light than small-diameter objectives. More light can pass through a large window than through a small one.

Resolving Power (Resolution)
The ability of a lens or lens system to form an image in which fine subject details are clearly differentiated. The higher the resolving power, or resolution, the more precisely the subject detail is rendered.

Reticle
In a rifle or handgun scope, the reticle is an aiming reference consisting of crosswires, dot, pointed post or other distinct shape that appears superimposed on the field of view. The reticle is positioned within the optical system to coincide with the plane of focus of the objective lens or lens group. Some scope sights are offered with a choice of reticle styles to accommodate subjective preferences and/ or match function to application. As a rule, to which there are exceptions, hunting-style scope sights have relatively bold reticles that aid rapid aiming, while target and varmint models feature finer reticles that subtend less of the target and may be less prominent, but are conducive to precise shot placement when aiming carefully and deliberately. Some spotting scopes and binoculars incorporate a scaled reticle intended to aid in estimating target distance or size.

Roof prism
Also called Dach prisms (Dach is the German word for roof), these relatively complex prisms erect the image in binoculars and “fold” the light path to allow shortening the barrels. Roof prisms are more difficult, and therefore more expensive, to manufacture than porno prisms, but they allow designing the binocular with compact, straight barrels, reducing size and often reducing weight. For a given level of quality, a pair of binoculars incorporating roof prisms is likely to cost more than one of similar power and light transmission that uses porno prisms. When minimizing instrument size and weight is not a major criterion for selection, overall optical and mechanical quality are more important than prism configuration.

Spherical Aberration
Causes varying degrees of image distortion. Light rays entering the outer portions of the lens are brought to a focus closer to the lens than light rays passing through the center of the lens. The resulting distortion is more likely to be noticed in photographs than when viewing through such a lens.

Twilight Factor
Most often associated with binoculars, the twilight factor is a numerical expression of the telescopic effect in dim light. It may also be calculated for scope sights. The twilight factor is derived by multiplying the magnification, or power, by the useful objective diameter, in millimetres, and then extracting the square root. In the case of a 4X scope sight with a 40 mm objective, the calculation would be 4 x 40 = V ‘160 = 12.64911. The twilight factor assumes realistically that in dim light, all other factors being equal, viewing instruments with higher magnifications and larger objectives will outperform those instruments with lower power and lesser light gathering capability.

Variable Power
Variable-power scope sights, spotting scopes and binoculars have a control that allows the user to adjust the magnification over a predetermined range, as with a photographic “zoom” lens.

Wide-Field
Wide-field viewing or sighting optics provide a broader field of view at any given distance than would a conventional non-wide-field instrument of the same magnification.

Sighting And Glasses
Shooting with protective glasses presents a different sight picture than without glasses because of the intervening layer of plastic or glass. Many shooters have noted that there is a displacement of objects in the field of view and believe the use of protective lenses introduces a sighting error.
Instead, as the diagram indicates, the direction of the gun is determined by the alignment of the sights with the target. The eye only sees the alignment after it has taken place. Even if there were a mirror or prism behind the sights, so that the sight picture could be viewed from entirely around a corner, it would make no difference. By the same reasoning, this principle also applies to optical sights as well as iron sights.

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