The diameter of the field in an optical microscope is expressed by the field-of-view number, or simply the field number, which is the diameter of the view field in millimeters measured at the intermediate image plane.

In most cases, the eyepiece field diaphragm opening diameter determines the view field size. The field size in the specimen plane is then defined as the field number divided by the magnification of the objective:

If an auxiliary lens is inserted between the objective and eyepiece, the magnification factor of this lens should also be employed in the equation by multiplication with the objective magnification (prior to the division operation). Although the field number is usually limited by the magnification and field diaphragm (stop) size of the eyepiece, there is clearly a limit that is also imposed by the design of the objective lens system. In early microscope objectives, the maximum usable field diameter tended to be about 18 millimeters or considerably less, but with modern plan apochromats and other specialized flat-field objectives, the maximum usable field can sometimes exceed 28 millimeters.

In modern microscope eyepieces, the field diaphragm either precedes the optical system or is located between the lens element groups, as illustrated in Figure 1. This figure presents cutaway diagrams of a Ramsden and Huygens eyepiece showing ray traces through the field lens, eye lens, and field diaphragm. The Ramsden eyepiece (illustrated on the right in Figure 1) has an optical system consisting of two plano-convex lenses having a fixed separation distance according to their focal lengths. The first lens is termed the field lens because it is closer to the plane where the intermediate image is formed in the microscope. The field diaphragm is located between the tube opening and the field lens in the Ramsden eyepiece design.

In all modern eyepiece designs, the field lens is placed a sufficient distance away from the intermediate image plane to ensure that dust or other surface debris and defects on the lens surface are not visualized along with the specimen. The top lens in both eyepiece designs is referred to as the eye lens because it is closest to the eye of the observer.

The Huygens eyepiece, illustrated on the left in Figure 1, is designed with the field lens positioned before the intermediate image plane, which coincides with the field diaphragm. Unlike the Ramsden eyepiece, the Huygens design has a focal point (at the intermediate image plane) residing between the eye and field lenses. In this case, the eye lens acts as a loupe to magnify the intermediate image.

The field number of typical eyepieces varies between 6 and 28 millimeters and (in general) decreases with the magnification of the eyepiece. For example, an eyepiece having a magnification of 10x typically has a field number ranging between 16 and 18 millimeters, while a lower magnification eyepiece (5x) has a field number of about 20 millimeters. Presented in Figure 2 is a comparison between the view fields available with similar eyepieces, one having a field number of 20 and the other a field number of 26. Note the greater range of specimen features visible through the eyepiece having the larger field number.

The size of the eyepiece field diaphragm opening is also dependent upon the correction for off-axial aberrations (coma, astigmatism, and lateral chromatic) of the objective. Recent eyepieces feature highly corrected glass that enables wide-field designs having field numbers of 26 millimeters and greater.