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. Inscriptions on the side of the eyepiece describe its particular characteristics and function. 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, 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. The best-quality digital cameras tend to have large lenses, such as the Canon EOS series of digital SLRs. Attempting afocal photomicrography through a normal eyepiece necessarily imposes unacceptable vignetting on such cameras. The solution is to replace the normal eyepiece with a “photo eyepiece”, which is a larger eyepiece designed to examine the same virtual image produced by the microscope objective as the normal eyepiece, but with apertures and focal lengths consistent with the entrance pupils of large camera lenses. This is several times more expensive than a simple afocal adapter, since the mechanical body is many times larger than the afocal adapter, and furthermore, sophisticated optical elements are required. There are photos showing a typical photo eyepiece which we designed and fabricated to adapt a 58mm camera thread to a 23mm eyepiece tube. This will couple many digital SLRs such as the Canon EOS series to the most common eyepiece standard. The optics are a 4-element symmetric (Plossl) design optimized for microscopy. This prototyping-styled design provides thumbscrew locks for the optical components, which allows you to evaluate various optical designs by easily switching various lens elements. The fixed design employs permanent retaining rings instead of thumbscrews.