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