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

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)

**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**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.

From Ruzin, 1999. Plant Microtechnique and Microscopy. Oxford University Press.