microscopic eyepiece 10x

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.