Page 22 - Fundamentals of Light Microscopy and Electronic Imaging

APERTURE AND IMAGE PLANES IN A FOCUSED, ADJUSTED MICROSCOPE 5

fraction plane viewed at this location is called the conoscopic image. In this text we refer
to the two viewing modes as the normal and aperture viewing modes and do not use the
terms orthoscopic and conoscopic, although they are common in other texts.

Note: Using an Eyepiece Telescope to View
the Objective Back Aperture

An aperture is a hole or opening in an opaque mask designed to eliminate stray light
from entering the light path, and most field and aperture planes of a microscope con-
tain apertures. A fixed circular aperture is found at or near the rear focal plane of the
objective lens. (The precise location of the back focal plane is a function of the focal
length of the lens; for objectives with short focal lengths, the focal plane is located
inside the lens barrel.) The aperture mask is plainly visible at the back surface of the
objective lens (Fig. 1-3). We refer to this site frequently in the text.
The eyepiece telescope (sometimes called a phase or centering telescope) is a special
focusable eyepiece that is used in place of an ocular to view the back aperture of the
objective lens and other aperture planes that are conjugate to it. To use the telescope,
remove the eyepiece, insert the eyepiece telescope, and focus it on the circular edge
of the objective back aperture. Some microscopes contain a built-in focusable tele-
scope lens called a Bertrand lens that can be conveniently rotated into and out of the
light path as required.

The identities of the sets of conjugate focal planes are listed here, and their loca-
tions in the microscope under conditions of Koehler illumination are shown in Figure
1-4. The terms front aperture and back aperture refer to the openings at the front and
back focal planes of a lens from the perspective of a light ray traveling from the lamp to
the retina. Knowledge of the location of these planes is essential for adjusting the micro-
scope and for understanding the principles involved in image formation. Indeed, the
entire design of a microscope is based on these planes and the user’s need to have access
to them. Taken in order of sequence beginning with the light source, they are as follows:

Field Planes Aperture Planes (aperture view
(normal view through the eyepieces) through the eyepiece telescope)
• lamp (field) diaphragm • lamp filament
• object or field plane • front aperture of condenser (condenser
• real intermediate image plane diaphragm)
(eyepiece field stop) • back aperture of objective lens
• retina or camera face plate • exit pupil of eyepiece (coincident with
the pupil of the eye)

The exit pupil of the eyepiece, which occupies the location of one of the aperture
planes, is the disk of light that appears to hang in space a few millimeters above the back
lens of the eyepiece; it is simply the image of the back aperture of the objective lens.
Normally we are unaware that we are viewing four conjugate field planes when looking
through the eyepieces of a microscope. As an example of the simultaneous visibility of
conjugate focal planes, consider that the image of a piece of dirt on a focused specimen
could lie in any one of the four field planes of the microscope: floaters near the retina,