Eyepieces (Oculars)

The eyepiece, or ocular lens, is the part of the microscope that magnifies the image produced by the microscope’s objective so that it can be seen by the human eye. In this resource we will look at the different types of eyepieces, their components, how they work, and how to use them.

Eyepiece vs. Ocular Lens

Eyepieces work in combination with microscope objectives to further magnify the intermediate image so that specimen details can be observed. Oculars, or ocular lenses, are alternative names for eyepieces. To maintain consistency during this discussion, we will refer to all oculars and ocular lenses as eyepieces.

To achieve the best results in microscopy, combine objectives with eyepieces that are appropriate for the correction and objective type. The basic anatomy of a typical modern eyepiece is illustrated in Figure 1 below. Inscriptions on the side of the eyepiece describe its characteristics and functions.

How to Read Eyepiece Inscriptions

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 on the manufacturer, to indicate a high-eyepoint focal point that enables microscopists to wear glasses while viewing samples.

Other inscriptions often found on eyepieces include:

• WF for widefield
• UWF for ultra-widefield
• SW and SWF for super widefield
• HE for high eyepoint
• 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 flatfield objectives are sometimes labeled plan-comp.

The eyepiece magnification of the eyepieces in Figure 1 is 10X, as indicated on the housing. 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 with rubber eyecups 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.

Types of Simple Eyepieces: Negative, Positive, and Modified

There are two major types of eyepieces that are grouped according to lens and diaphragm arrangement: negative eyepieces (or Huygenian eyepieces) with an internal diaphragm and positive eyepieces (or Ramsden eyepieces) that have a diaphragm below the lenses of the eyepiece.

Negative eyepieces have two lenses:

• Upper lens, which is closest to the observer's eye, is called the eye lens
• Lower lens (beneath the diaphragm) is often termed the field lens

In their simplest form, both the eye and field lenses are plano-convex, with convex sides facing the specimen. About midway between these lenses is a fixed circular opening or internal diaphragm. The size of the diaphragm defines the circular field of view that is observed when you look into the microscope.

Learn more about the difference between negative and positive eyepieces below.

What Is a Huygenian Eyepiece?

The simplest negative eyepiece design, often termed the Huygenian eyepiece (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 together to make the eye lens. If an unknown eyepiece has only the magnification inscribed on the housing, it is most likely a Huygenian eyepiece and is best suited for use with achromatic objectives of 5X to 40X magnification.

What Is a Ramsden Eyepiece?

The other main type of simple 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 toward 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.

Modified Simple Eyepieces

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 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.

Compensating Eyepieces

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 combined with high magnification achromatic objectives or fluorite or apochromatic objectives. To fix this issue, manufacturers produce compensating eyepieces that introduce an equal but opposite chromatic error in the lens elements.

Compensating eyepieces may be 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 (e.g., Olympus objectives) or corrected in the tube lens.

Compensating eyepieces play a crucial role to help eliminate residual chromatic aberrations inherent in the design of highly corrected objectives. As a result, it is preferable that the microscopist uses the compensating eyepieces designed by a particular manufacturer to accompany that manufacturer's higher-corrected objectives. Using an incorrect eyepiece with an apochromatic objective designed for a finite (160 or 170 mm) tube length application results in dramatically increased contrast with red fringes on the outer diameters and blue fringes on the inner diameters of the specimen details. Additional problems arise from a limited flatness of the viewfield in simple eyepieces, even those corrected with eye-lens doublets.

Advanced Eyepieces

More advanced eyepiece designs resulted in the Periplan eyepiece that is illustrated in Figure 4 above. This eyepiece contains seven lens elements 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 widefield eyepieces (illustrated in Figure 1) that increase the viewable area of the specimen by as much as 40 percent. Because eyepiece-objective correction techniques vary from manufacturer to manufacturer, it is important to use only the eyepieces recommended by a specific manufacturer for use with their objectives.

How to Choose the Right Eyepiece

Our recommendation is to carefully choose the objective first, then purchase an eyepiece designed to work 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 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

Table 1

The properties of several common commercially available eyepieces (manufactured by Olympus) are listed according to type in Table 1. The three major types of eyepieces listed in Table 1 are finder, widefield, and super widefield.

Note that the terminology used by various manufacturers can be confusing. Pay careful attention to brochures and microscope manuals to choose the correct eyepieces for a specific objective.

In Table 1, the abbreviations that designate widefield 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 range from 14 to 26.5, depending on the application. The diopter adjustment is approximately the same for all eyepieces, and many also contain either a photomask or micrometer reticle.

High Eyepoint Eyepieces

Light rays emanating from the eyepiece intersect at the exit pupil or eyepoint, often referred to as the Ramsden disk, where the pupil of the microscopists eye should be placed in order 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 issue, manufactures have designed high eyepoint eyepieces 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. These eyepieces are often designated with the inscription H somewhere on the eyepiece housing, either alone or in combination with other abbreviations.

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 comfortable.

Viewfield Diameter

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). 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
(SWF 10X Eyepiece)

Table 2

Range of Useful Magnification

Take care when choosing eyepiece/objective combinations to help 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. Another choice for the same magnification would be a 10X eyepiece with a 25X objective. Because the 25X objective has a higher numerical aperture (about 0.65 NA) than the 10X objective (about 0.25 NA) and numerical aperture values define an objective's resolution, the latter choice is ideal. 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 other 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 1,000 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 below lists the common objective/eyepiece combinations that lie in the range of useful magnification.

Range of Useful Magnification
(500–1000 × NA of Objective)

Table 3

Measuring Graticules

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 a 35 mm format. Other film formats (120 mm and 4 × 5 in.) are delineated by sets of corners within the larger 35 mm 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, so consult the manufacturers of microscopes and optical accessories to determine the types and availability of these useful measuring devices.

Filar Micrometers

For highly accurate measurements, a filar micrometer (similar to the one illustrated in Figure 6) is used. This micrometer replaces the conventional eyepiece and offers 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, as shown in 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, as shown in 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.

Movable Pointers

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 to indicate certain features of a specimen, especially when a microscopist is teaching students about specific features. Most eyepiece pointers can be rotated in a 360° angle around the specimen, and more advanced versions can translate across the viewfield.

Photo Eyepieces and Projection Lenses

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 incapable 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 help ensure true reproduction of color in color photomicrography. Magnification factors in photomicrography projection lenses range from 1X to about 5X. These lenses can be interchanged to adjust the size of the final image in the photomicrograph.

Focusing Telescopes

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 enables 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.

Frequently Asked Questions

What is the ocular lens on a microscope?

The ocular lens may refer to the eyepiece as a whole or specifically to the eye lens—the lens closest to the eye.

What does the ocular lens do on a microscope?

The ocular lens magnifies the image produced by the objective so that the microscope user can see it.

How do I select the right eyepiece?

There are many factors that go into selecting an eyepiece. The important thing to keep in mind is that your eyepiece and objective should be compatible. Our recommendation is to carefully choose the objective first, then purchase an eyepiece designed to work with the objective.