Eyepiece | Define Eyepiece at Dictionary.com eyepiece noun 1. the lens or combination of lenses in an optical instrument through which the eye views the image formed by the objective lens or lenses; ocular. Origin of eyepiece Examples from the Web for eyepiece Expand Historical Examples Insert the eyepiece micrometer inside the ocular and adjust the stage micrometer on the stage of the microscope. The Elements of Bacteriological Technique John William Henry Eyre A man sat in a chair on the platform at the microscope's eyepiece. The Hell Ship Raymond Alfred Palmer She is our eyepiece to keep us cognizant of our opportunities. A Labrador Doctor Wilfred Thomason Grenfell She turned back to the microscope and bent over the eyepiece. The telescope consists of two lenses, the eyepiece and the objective. Physics Willis Eugene Tower The Metal Moon Everett C. Smith British Dictionary definitions for eyepiece Expand noun 1. the lens or combination of lenses in an optical instrument nearest the eye of the observer Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd.

1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Ocular lens | definition of Ocular lens by Medical dictionary Ocular lens | definition of Ocular lens by Medical dictionary eyepiece [i´pēs] the lens or system of lenses of a microscope (or telescope) nearest the user's eye, serving to further magnify the image produced by the objective. Called also ocular . eye·piece (ī'pēs), The compound lens at the end of the microscope tube nearest the eye; it magnifies the image made by the objective lens. eyepiece /eye·piece/ (-pēs) the lens or system of lenses of a microscope (or telescope) nearest the user's eye, serving to further magnify the image produced by the objective. eyepiece (ī′pēs′) n. The lens or lens group closest to the eye in an optical instrument; an ocular. Also called eyeglass.

eye·piece (ī'pēs) The compound lens at the end of the microscope tube nearest the eye; it magnifies the image made by the objective. eyepiece The lens or combination of lenses in an optical instrument (microscope, telescope, etc.) through which the observer views the image formed by the objective. The most common eyepieces are composed of two single lenses or two doublets: the lens or doublet nearer the eye is called the eye lens and the one nearer the objective is called the field lens. The role of the eyepiece is to magnify the image and to reduce the aberrations of the image formed by the objective. Syn. eye lens; eyeglass; ocular. Huygens' eyepiece Negative eyepiece used commonly in microscopes. It consists of two planoconvex lenses mounted with their plane surfaces facing the eye. In the most common type the eye lens has a focal length half that of the field lens and the separation is equal to half the sum of the two focal lengths.

negative eyepiece Eyepiece made up of two lenses, in which the first principal focus of the eyepiece lies between the two lenses, such as in a Huygens' eyepiece. orthoscopic eyepiece An eyepiece corrected for distortion, and which provides a wide field of view and high magnification. It consists of a triplet field lens and a single eye lens. It is used in high-power telescopes and range finders. See triplet . positive eyepiece Eyepiece made up of two lenses in which the first principal focus of the eyepiece lies in front of the field lens, such as in a Ramsden eyepiece. Ramsden eyepiece Positive eyepiece consisting of two planoconvex lenses mounted with their convex side facing each other and having equal focal lengths. The lenses are usually separated by two-thirds the focal length of either. eyepiece the lens or system of lenses of a microscope (or telescope) nearest the user's eye, serving to further magnify the image produced by the objective.

Your Complete Telescope Eyepiece Guide from Sky & Telescope field of view (degrees) = Eyepiece Types Some names, like Ultrawide, give a clue to an eyepiece's principal characteristics. Others have classic names like Kellner, Orthoscopic, Plössl, and Erfle, or more specific modern variants like Nagler and Lanthanum. Optical aberrations (deviations from the "ideal" form) are better corrected in some designs than others, but like everything in life, the higher the price, the better the quality. Even so, eyepieces with desirable characteristics still can be obtained at reasonable cost once you know what to look for. At the budget end of the market, three-element (meaning they have three lenses inside) eyepieces labeled Kellner or "MA" (modified achromat) can turn in a good performance with scopes of long focal ratio, such as Schmidt-Cassegrains and traditional refractors. They do not work well with telescopes of short focal ratio. For optically "fast" Newtonians and the like, better results will be obtained with four-element designs, such as the slightly out-of-fashion but otherwise desirable Orthoscopic, or the ubiquitous Plössl.

The latter is a good all-rounder, particularly when antireflection multicoated. A Plössl can deliver well-corrected, wide fields of view with good eye relief — meaning the eye can be positioned at a comfortable distance behind the rear lens and still see the whole field of view. It is worth pointing out that observers who wear glasses to correct for simple long- or short-sightedness (no astigmatism) don't need to use them at the telescope; a twist of the focuser will remedy that. High-power lunar and planetary viewing entails the use of short-focal-length eyepieces, which can be a problem owing to the small eye relief of conventional designs. This has spawned the development of six- to eight-element designs that combine comfortable eye relief and wide apparent fields across the focal-length range. They may feature exotic rare-earth glass elements to reduce optical aberrations still further. Many regard these Lanthanum and Ultrawide designs as the pinnacle of eyepiece evolution (and rightly so), but for many people the price will be a barrier — and their physical size and weight may be an issue for delicately balanced small scopes.

Barlow Lenses While the Barlow is not strictly an eyepiece, our discussion would not be complete without mentioning this enormously valuable accessory. A Barlow is inserted into the telescope's focuser before the eyepiece and instantly doubles (or triples, in some instances) the magnification. At the expense of a small loss of light, this very useful device can double your eyepiece investment by making each perform at two powers. If you are contemplating purchasing a Barlow with a set of eyepieces, give some thought to staggering the focal lengths so that they are not simple multiples of one another: a 10 mm eyepiece is largely redundant if you use a 20 mm and a Barlow. Look for "achromatic" and "multi-coated" specifications when shopping around. Get expert advice on eyepieces and other telescope equipment when you subscribe to Sky & Telescope ! CATEGORIES Choosing Your Equipment — A Guide for Astronomers , Equipment: Guides & Recommendations All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments.

Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy . COMMENT Microscope Eyepieces The Microscope Eyepiece or Ocular Simple eyepieces are constructed of two Plano-convex lenses (A, B). More corrected lenses consist of three or more lenses, with at least one as an achromatic doublet (C). All eyepieces also have an internal aperture, that is used to reduce aberrations, but that limits the field of view. Historically, the earliest eyepiece is the Huygenian eyepiece (A; named after Christian Huygens ) has the aperture (or diaphragm) placed between the two lenses. A later invention, the Ramsden eyepiece (named after Jesse Ramsden ) has the aperture placed before the first lens (B). Both eyepiece designs suffer from Chromatic aberration. The Ramsden yields a better image however. A variation of the Ramsden, the Kellner eyepiece (after Carl Kellner ) replaces the Eye Lens with an achromatic doublet.

The Kellner lens has good chromatic correction, and is reasonably inexpensive. Modern eyepieces are a variation of the Kellner design. With increased use of achromatic doublets and triplets. Chromatic aberration is eliminated and the field flattened. Power (P, magnification of an eyepiece is defined as D/focal length; where D = the closest distance of distinct vision, or 250mm (average in humans). Magnification of a compound microscope is therefore: The microscope eyepiece, or ocular, magnifies the real image projected by the objective lens to form a large virtual image at the distance of most distinct vision (25 cm). As with objective lenses, modern eyepieces are manufactured of multiple optical components for correction of lens aberrations. In many cases the eyepiece may also be matched to a particular type of objective lens for further correction of objective aberrations. When this is the case the eyepiece is referred to as a compensating eyepiece. Compensating eyepieces correct primarily chromatic difference of magnification (lateral chromatic aberration) and field curvature induced by aberrations in the objective lens elements.

The basic design of a microscope eyepiece consists of two lenses (or lens doublets) separated by an air space the length of half the sum of the focal lengths of the two lenses. The lens closest to the eye is called the eye lens and the other lens (closest to the objective) is called the field lens. The result of having two lenses within the eyepiece is to reduce the size of the intermediate image and thus allow the observation of a larger field of view. The eyepiece focuses the real image (from the objective) outside of the eye lens at a position called the eyepoint or exit pupil, which corresponds to the position of the focal point of the eye. Eye relief is a parameter of eyepiece design and determines the distance between the exit pupil and the eye. It must be greater than 5–7 mm to avoid observer difficulties and is usually 7–13 mm. High eyepoint oculars, designed for glasses wearers, have an eye relief of 15–20 mm. The diameter of the eyepiece aperture (in mm) is called the Field-of-view-number, or Field Number (FN). Remember that the eyepiece aperture is located at the intermediate image plane.

The FN is often embossed on the barrel of the eyepiece. Given the FN one can calculate the diameter of the sample imaged field: Sample View = FN / (Mobj x Mtube) where FN - Field number (in mm) Mobj = magnification of the objective Mtube = magnificatio of the tube lens (often 1.25) Conventions of eyepiece markings Grad markings. On a binocular microscope the eye lens of each eyepiece may be focused so that they may be customized to the operators eyesight. Usually there is a “0” position that represents the unmodified, or “zero focus” position. Since focusing eyepieces are, in essence, equivalent to eyeglasses, the microscopist need not wear glasses when using the instrument. If, however, the operator’s eyesight is astigmatic, the operator must wear correcting eyeglasses when using the microscope. Red dot. This is the “zero focus” position if an integrated reticle is inserted in the eyepiece. Field of View: Lateral dimension of the sample that is imaged by the eyepiece.

FOV = eyepiece FN (in mm)/(objective magnification x tube lens magnification) Field Number: Measure of the imaged Field of View. FN is the diameter (in mm) of the image at the intermediate image plane. This is usually the aperture in the eyepiece. Eye relief: Distance from the eyepiece to the point where the exit pupil is the same size as the eye's pupil. Eyerelief can be up to 20mm. High eyepoint. These eyepieces are designed in such a way that the exit pupil is further away from the eye lens than standard eyepieces (16 mm or more) and thus these eyepieces are well suited for eyeglasses wearers. With this eyepiece the field of view also is usually wider (up to 54°). The graphic designation is a pair of eye glasses. KPL. These eyepieces are designed for photomicrography (digital imaging) and have a very high eyepoint. Another name for this type of eyepiece is projection lens. Pl. “Plan”; flat field-corrected compensating eyepieces. C, Comp.

These eyepieces correct aberrations imparted by the objective. Such compensating eyepieces reduce chromatic difference in magnification and field curvature from the objective by introducing equal and opposite aberrations. 10x/25. This refers to the eyepiece magnification (10x) and a field-of-view number of 25 (high eyepoint; 53° field of view). Eyepieces are usually 6.3x, 8x, 10x, 12x, 16x, and infrequently 20x. As the magnification of the eyepiece increases, the field of view decreases. Pol. These eyepieces are constructed of strain-free glass to be used in polarization microscopy. Terms describing eyepieces Objective lens - definition of objective lens by The Free Dictionary Objective lens - definition of objective lens by The Free Dictionary Noun 1.

objective lens - the lens or system of lenses in a telescope or microscope that is nearest the object being viewed object glass , object lens , objective compound microscope - light microscope that has two converging lens systems: the objective and the eyepiece lens , lens system , lense - a transparent optical device used to converge or diverge transmitted light and to form images optical telescope - an astronomical telescope designed to collect and record light from cosmic sources Want to thank TFD for its existence? Tell a friend about us , add a link to this page, or visit the webmaster's page for free fun content . Link to this page: telescope References in periodicals archive ? By combining a 25mm objective lens with a smaller ocular lens (approximately 20mm) Trijicon was able to reduce the tunneling effect you see in a tube red dot sight. Trijicon MRO (miniature rifle optic) Where n is the index of refraction of the medium surrounding the objective lens and 0 is half the maximum angle of entry by light into the lens as measured from its focal point P, as shown in FIGURE 5.

Objective Lens Markets in the World to 2018 - Market Size, Trends, and Forecasts A and B, Tumor with 3+ membrane staining, visible using a35 objective lens (A) and confirmed using a310 objective lens (B). Copyright © 2003-2017 Farlex, Inc Disclaimer All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. Compound Microscope The compound microscope consists of two optical components (thus the term compound): the objective lens system, which has a very short focal distance and is placed very close to the object; and the ocular or eyepiece system, which has a longer focal length, lower magnification; and which further magnifies and projects the image onto the retina of the eye. The objective lens projects a real image (the intermediate image) of the object up in the body of the microscope onto the objective conjugate image plane or primary image plane.

The distance from the back focal plane of the objective to the primary image plane within the microscope body is referred to as the optical tube length. Depending on the type of microscope and eyepiece, the primary image plane may be either within the microscope body itself or within the eyepiece (see Infinity corrected optics). The objective lens forms a real image in the microscope body that acts as the object for the ocular lens. The real image becomes the object for the eye itself and is projected onto the retina. The ocular in turn acts as a simple magnifier to form a large virtual image at the distance of most distinct vision (25 cm) from the eye. The objective and ocular lenses act in series. Thus, the total magnification in a compound microscope is a function of the objective magnification multiplied by the eyepiece magnification. The Compound Microscope The advantage of having a compound lens system as opposed to a simple magnifier is that much higher magnifications can be achieved with two lenses rather than one.

To achieve high magnification with single-lens magnifiers (a magnifying lens, for example) the lens itself must be placed close to the eye to increase the subtending angle, and thus the size, of the projected image on the retina The eye then focuses on the large virtual image formed by the positive lens magnifier. Modern magnifying lenses, as well as Leeuwenhoek's first magnifier, worked in just such a manner. A problem arises when very high magnifications are required because lenses cannot be placed comfortably close enough to the eye to result in a large subtending object-eye angle. The single-lens, double-convex magnifier of van Leeuwenhoek achieved at most 275x magnification. Light from the Eyepiece takes two separate paths depending on its origin. If it is sample light (the real image of the sample) the rays emerge from the eyepiece nearly parallel. At their most focused point they form a circle of light approximately 3mm in diameter. This circle (or the circle of light from any lens) is called the Exit Pupil. The distance from the front glass surface to the exit pupil is called the Eye Relief.

This is usually 15-17mm, but can be further in High-Eyepoint oculars. A compound microscope can be viewed as having two simultaneous, but mutually exclusive sets of conjugate planes. One set, the Sample Image Planes, consist of the following: A: Field Iris, B: Sample Plane, C: Intermediate Image, and D: Retina. The other important set, the Sample Illuminating Planes, consist of E: Light source (Filament), F: Condenser Iris, G: Objective back focal plane, H: Exit Pupil. Eyepiece lens | definition of Eyepiece lens by Medical dictionary Eyepiece lens | definition of Eyepiece lens by Medical dictionary Related to Eyepiece lens: objective lens , Ocular lens eyepiece [i´pēs] the lens or system of lenses of a microscope (or telescope) nearest the user's eye, serving to further magnify the image produced by the objective. Called also ocular . eye·piece (ī'pēs), The compound lens at the end of the microscope tube nearest the eye; it magnifies the image made by the objective lens.

eyepiece /eye·piece/ (-pēs) the lens or system of lenses of a microscope (or telescope) nearest the user's eye, serving to further magnify the image produced by the objective. eyepiece (ī′pēs′) n. The lens or lens group closest to the eye in an optical instrument; an ocular. Also called eyeglass. eye·piece (ī'pēs) The compound lens at the end of the microscope tube nearest the eye; it magnifies the image made by the objective. eyepiece The lens or combination of lenses in an optical instrument (microscope, telescope, etc.) through which the observer views the image formed by the objective. The most common eyepieces are composed of two single lenses or two doublets: the lens or doublet nearer the eye is called the eye lens and the one nearer the objective is called the field lens. The role of the eyepiece is to magnify the image and to reduce the aberrations of the image formed by the objective. Syn. eye lens; eyeglass; ocular. Huygens' eyepiece Negative eyepiece used commonly in microscopes.

It consists of two planoconvex lenses mounted with their plane surfaces facing the eye. In the most common type the eye lens has a focal length half that of the field lens and the separation is equal to half the sum of the two focal lengths. negative eyepiece Eyepiece made up of two lenses, in which the first principal focus of the eyepiece lies between the two lenses, such as in a Huygens' eyepiece. orthoscopic eyepiece An eyepiece corrected for distortion, and which provides a wide field of view and high magnification. It consists of a triplet field lens and a single eye lens. It is used in high-power telescopes and range finders. See triplet . positive eyepiece Eyepiece made up of two lenses in which the first principal focus of the eyepiece lies in front of the field lens, such as in a Ramsden eyepiece. Ramsden eyepiece Positive eyepiece consisting of two planoconvex lenses mounted with their convex side facing each other and having equal focal lengths. The lenses are usually separated by two-thirds the focal length of either.

eyepiece the lens or system of lenses of a microscope (or telescope) nearest the user's eye, serving to further magnify the image produced by the objective. Olympus Microscopy Resource Center | Anatomy of a Microscope - Eyepieces (Oculars) Software Downloads Eyepieces (Oculars) Eyepieces work in combination with microscope objectives to further magnify the intermediate image so that specimen details can be observed. Oculars is an alternative name for eyepieces that has been widely used in the literature, but to maintain consistency during this discussion we will refer to all oculars as eyepieces. Best results in microscopy require that objectives be used in combination with eyepieces that are appropriate to the correction and type of objective. The basic anatomy of a typical modern eyepiece is illustrated in Figure 1. Inscriptions on the side of the eyepiece describe its particular characteristics and function. The eyepieces illustrated in Figure 1 are inscribed with UW, which is an abbreviation for the Ultra Wide viewfield. Often eyepieces will also have an H designation, depending upon the manufacturer, to indicate a high-eyepoint focal point that allows microscopists to wear glasses while viewing samples.

Other inscriptions often found on eyepieces include WF for Wide-Field; UWF for Ultra Wide-Field; SW and SWF for Super Wide-Field; HE for High Eyepoint; and CF for eyepieces intended for use with CF corrected objectives. Compensating eyepieces are often inscribed with K, C, or comp as well as the magnification. Eyepieces used with flat-field objectives are sometimes labeled Plan-Comp. The eyepiece magnification of the eyepieces in Figure 1 is 10x (indicated on the housing), and the inscription A/24 indicates the field number is 24, which refers to the diameter (in millimeters) of the fixed diaphragm in the eyepiece. These eyepieces also have a focus adjustment and a thumbscrew that allows their position to be fixed. Manufactures now often produce eyepieces having rubber eye-cups that serve both to position the eyes the proper distance from the front lens, and to block room light from reflecting off the lens surface and interfering with the view.

There are two major types of eyepieces that are grouped according to lens and diaphragm arrangement: the negative eyepieces with an internal diaphragm and positive eyepieces that have a diaphragm below the lenses of the eyepiece. Negative eyepieces have two lenses: the upper lens, which is closest to the observer's eye, is called the eye-lens and the lower lens (beneath the diaphragm) is often termed the field lens. In their simplest form, both lenses are plano-convex, with convex sides "facing" the specimen. Approximately mid-way between these lenses there is a fixed circular opening or internal diaphragm which, by its size, defines the circular field of view that is observed in looking into the microscope. The simplest negative eyepiece design, often termed the Huygenian eye-piece (illustrated in Figure 2), is found on most teaching and laboratory microscopes fitted with achromatic objectives. Although the Huygenian eye and field lenses are not well corrected, their aberrations tend to cancel each other out. More highly corrected negative eyepieces have two or three lens elements cemented and combined together to make the eye lens.

If an unknown eyepiece carries only the magnification inscribed on the housing, it is most likely to be a Huygenian eyepiece, best suited for use with achromatic objectives of 5x-40x magnification. The other main kind of eyepiece is the positive eyepiece with a diaphragm below its lenses, commonly known as the Ramsden eyepiece, as illustrated in Figure 2 (on the left). This eyepiece has an eye lens and field lens that are also plano-convex, but the field lens is mounted with the curved surface facing towards the eye lens. The front focal plane of this eyepiece lies just below the field lens, at the level of the eyepiece diaphragm, making this eyepiece readily adaptable for mounting reticles. To provide better correction, the two lenses of the Ramsden eyepiece may be cemented together. A modified version of the Ramsden eyepiece is known as the Kellner eyepiece, as illustrated on the left in Figure 3.

These improved eyepieces contain a doublet of eye-lens elements cemented together and feature a higher eyepoint than either the Ramsden or Huygenian eyepiece as well as a much larger field of view. A modified version of the simple Huygenian eyepiece is also illustrated in Figure 3, on the right. While these modified eyepieces perform better than their simple one-lens counterparts, they are still only useful with low-power achromat objectives. Simple eyepieces such as the Huygenian and Ramsden and their achromatized counterparts will not correct for residual chromatic difference of magnification in the intermediate image, especially when used in combination with high magnification achromatic objectives as well as any fluorite or apochromatic objectives. To remedy this, manufacturers produce compensating eyepieces that introduce an equal, but opposite, chromatic error in the lens elements. Compensating eyepieces may be either of the positive or negative type, and must be used at all magnifications with fluorite, apochromatic and all variations of plan objectives (they can also be used to advantage with achromatic objectives of 40x and higher).

In recent years, modern microscope objectives have their correction for chromatic difference of magnification either built into the objectives themselves ( Olympus and Nikon) or corrected in the tube lens (Leica and Zeiss). Compensating eyepieces play a crucial role in helping to eliminate residual chromatic aberrations inherent in the design of highly corrected objectives. Hence, it is preferable that the microscopist uses the compensating eyepieces designed by a particular manufacturer to accompany that manufacturer's higher-corrected objectives. Use of an incorrect eyepiece with an apochromatic objective designed for a finite (160 or 170 millimeter) tube length application results in dramatically increased contrast with red fringes on the outer diameters and blue fringes on the inner diameters of specimen detail. Additional problems arise from a limited flatness of the viewfield in simple eyepieces, even those corrected with eye-lens doublets. More advanced eyepiece designs resulted in the Periplan eyepiece that is illustrated in Figure 4 above. This eyepiece contains seven lens elements that are cemented into a single doublet, a single triplet, and two individual lenses.

Design improvements in periplan eyepieces lead to better correction for residual lateral chromatic aberration, increased flatness of field, and a general overall better performance when used with higher power objectives. Modern microscopes feature vastly improved plan-corrected objectives in which the primary image has much less curvature of field than older objectives. In addition, most microscopes now feature much wider body tubes that have greatly increased the size of intermediate images. To address these new features, manufacturers now produce wide-eyefield eyepieces (illustrated in Figure 1) that increase the viewable area of the specimen by as much as 40 percent. Because the strategies of eyepiece-objective correction techniques vary from manufacturer to manufacturer, it is very important (as stated above) to use only eyepieces recommended by a specific manufacturer for use with their objectives. Our recommendation is to carefully choose the objective first, then purchase an eyepiece that is designed to work in conjunction with the objective. When choosing eyepieces, it is relatively easy to differentiate between simple and more highly compensated eyepieces.

Simple eyepieces such as the Ramsden and Huygenian (and their more highly corrected counterparts) will appear to have a blue ring around the edge of the eyepiece diaphragm when viewed through the microscope or held up to a light source. In contrast, more highly corrected compensating eyepieces with have a yellow-red-orange ring around the diaphragm under the same circumstances. Properties of Commercial Eyepieces 24 Table 1 The properties of several common commercially available eyepieces (manufactured by Olympus America, Inc.) are listed according to type in Table 1. The three major types of eyepieces listed in Table 1 are Finder, Wide Field, and Super Widefield. The terminology used by various manufacturers can be very confusing and careful attention should be paid to their sales brochures and microscope manuals to ensure that the correct eyepieces are being used with a specific objective. In Table 1, the abbreviations that designate wide field and super widefield eyepieces are coupled to their correction for high eyepoint, and are WH and SWH, respectively.

The magnifications are either 10x or 15x and the Field Numbers (discussed below) range from 14 to 26.5, depending upon the application. The diopter adjustment is approximately the same for all eyepieces and many also contain either a photomask or micrometer reticle. Light rays emanating from the eyepiece intersect at the exit pupil or eyepoint, often referred to as the Ramsden disc, where the pupil of the microscopists eye should be placed in order for her to see the entire field of view (usually 8-10 mm from the eye lens). By increasing the magnification of the eyepiece, the eyepoint is drawn closer to the upper surface of the eye lens, making it much more difficult for the microscopist to use, especially if they are wearing eyeglasses. To compensate for this, specially designed high eyepoint eyepieces have been manufactured that feature eyepoint distances approaching 20-25 mm above the surface of the eye lens. These improved eyepieces have larger diameter eye lenses that contain more optical elements and usually feature improved flatness of field.

Such eyepieces are often designated with the inscription "H" somewhere on the eyepiece housing, either alone or in combination with other abbreviations, as discussed above. We should mention that high-eyepoint eyepieces are especially useful for microscopists who wear eyeglasses to correct for near or far sightedness, but they do not correct for several other visual defects, such as astigmatism. Today, high eyepoint eyepieces are very popular, even with people who do not wear eyeglasses, because the large eye clearance reduces fatigue and makes viewing images through the microscope much more pleasurable. At one time, eyepieces were available in a wide spectrum of magnifications ranging from 6.3x to 25x and sometimes even higher for special applications. These eyepieces are very useful for observation and photomicrography with low-power objectives. Unfortunately, with higher power objectives, the problem of empty magnification becomes important when using very high magnification eyepieces and these should be avoided. Today most manufacturers restrict their eyepiece offerings to those in the 10x to 20x range.

The diameter of the viewfield in an eyepiece is expressed as a "field-of-view number" or field number (FN), as discussed above. Information about the field number of an eyepiece can yield the real diameter of the object viewfield using the formula: Viewfield Diameter = (FN) / (M(O) × M(T)) where FN is the field number in millimeters, M(O) is the objective magnification, and M(T) is the tube lens magnification factor (if any). Applying this formula to the Super Widefield eyepiece listed in Table 1, we arrive at the following for a 40x objective with a tube lens magnification of 1.25: FN = 26.5 / M(O) = 40 × M(T) = 1.25 = a viewfield diameter of 0.53 mm. Table 2 lists the viewfield sizes over the common range of objectives that would occur using this eyepiece. Viewfield Diameters 0.085 Table 2 Care should be taken in choosing eyepiece/objective combinations to ensure the optimal magnification of specimen detail without adding unnecessary artifacts.

For instance, to achieve a magnification of 250x, the microscopist could choose a 25x eyepiece coupled to a 10x objective. An alternative choice for the same magnification would be a 10x eyepiece with a 25x objective. Because the 25x objective has a higher numerical aperture (approximately 0.65) than does the 10x objective (approximately 0.25), and considering that numerical aperture values define an objective's resolution, it is clear that the latter choice would be the best. If photomicrographs of the same viewfield were made with each objective/eyepiece combination described above, it would be obvious that the 10x eyepiece/25x objective duo would produce photomicrographs that excelled in specimen detail and clarity when compared to the alternative combination. The "range of useful magnification" for an objective/eyepiece combination is defined by the numerical aperture of the system. There is a minimum magnification necessary for the detail present in an image to be resolved, and this value is usually rather arbitrarily set as 500 times the numerical aperture (500 × NA).

At the other end of the spectrum, the maximum useful magnification of an image is usually set at 1000 times the numerical aperture (1000 × NA). Magnifications higher than this value will yield no further useful information or finer resolution of image detail, and will usually lead to image degradation. Exceeding the limit of useful magnification causes the image to suffer from the phenomenon of "empty magnification", where increasing magnification through the eyepiece or intermediate tube lens only causes the image to become more magnified with no corresponding increase in detail resolution. Table 3 lists the common objective/eyepiece combinations that lie in the range of useful magnification. Range of Useful Magnification --- Table 3 Eyepieces can be adapted for measurement purposes by adding a small circular disk-shaped glass reticle (sometimes referred to as a graticule or reticule) at the plane of the field diaphragm of the eyepiece. Reticles usually have markings, such as a measuring rule or grid, etched onto the surface. Because the reticle lies in the same plane as the field diaphragm, it appears in sharp focus superimposed over the image of the specimen.

Eyepieces using reticles must contain a focusing mechanism (usually a helical screw or slider) that allows the image of the reticle to be brought into focus. Several typical reticles are illustrated in Figure 5 below. The reticle in Figure 5(a) is a common element of eyepieces intended to "frame" viewfields for photomicrography. The small rectangular element circumscribes the area that will be captured on film using 35 mm format. Other film formats (120 mm and 4 × 5 inch) are delineated by sets of "corners" within the larger 35mm rectangle. In the center of the reticle is a series of circles surrounded by four sets of parallel lines arranged in an "X" pattern. These lines are used to focus the reticle and image to be parfocal with the film plane in a camera back attached to the microscope. The reticle in Figure 5(b) is a linear micrometer that can be used to measure image distances, and the crossed micrometer in 5(c) is used with polarizing microscopes to locate the alignment of samples with respect to the polarizer and analyzer.

The grid illustrated in Figure 5(d) is used to partition a section of the viewfield for counting. There are many other variations of eyepiece reticles, and the reader should consult the many manufacturers of microscopes and optical accessories to determine the types and availability of these useful measuring devices. For highly accurate measurements a Filar Micrometer similar to the one illustrated in Figure 6 is used. This micrometer replaces the conventional eyepiece and contains several improvements over conventional reticles. In the filar micrometer, a reticle with a measuring scale (there are many variations in scale types) and a very fine wire is brought into focus with the specimen (Figure 6(b)). The wire is mounted so that it can be slowly moved across the viewfield by the calibrated thumbscrew located on the side of the micrometer (Figure 6(a)). One complete turn of the thumbscrew (divided into 100 equal divisions) equals the distance between two adjacent reticle marks. By slowly moving the wire from one position on the specimen image to another and taking note of the changes in thumb screw numbers, the microscopist has a much more accurate measurement of distance.

Filar micrometers (and other simple reticles) must be calibrated with a stage micrometer for each objective with which it will be used. Some eyepieces have a movable "pointer" located within the eyepiece and positioned so that it appears as a silhouette in the image plane. This pointer is useful when indicating certain features of a specimen, especially when a microscopist is teaching students about specific features. Most eyepiece pointers can be rotated in a 360 degree angle around the specimen and more advanced versions can translate across the viewfield. Manufacturers often produce specialized eyepieces, often termed photo eyepieces, that are designed to be used with photomicrography. These eyepieces are usually negative (Huygenian type) and are not capable of being used visually. For this reason, they are typically called projection lenses. A typical projection lens is illustrated in Figure 7 below. Projection lenses must be carefully corrected so that they will produce flat-field images, a definite "must" for accurate photomicrography. They are generally also color-corrected to ensure true reproduction of color in color photomicrography.

Magnification factors in photomicrography projection lenses range from 1x to about 5x, and these can be interchanged to adjust the size of the final image in the photomicrograph. Camera systems have become an integral part of the microscope and most manufacturers provide photomicrographic attachment cameras as an optional accessory. These advanced camera systems often feature motorized black boxes that store and automatically step through film frame-by-frame as photomicrographs are taken. A common feature of these integral camera systems is a beamsplitter focusing telescopic eyepiece (see Figure 8) that allows the microscopist to view, focus, and frame samples for photomicrography. This telescope contains a photomicrography reticle, similar to the one illustrated in Figure 5(a) that is inscribed with a rectangular element that circumscribes the area captured with 35 mm film, and also corner brackets for larger format films. For convenience in scanning and photographing samples, the microscopist can adjust the telescopic eyepiece so that it is parfocal with the ocular eyepieces to make it easier to frame and take photomicrographs.

Contributing Authors Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747. Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

Question: In telescopes and microscopes, what is the alternative name for the ocular lens?
Answer: Eyepiece