Page 67 - Fundamentals of Light Microscopy and Electronic Imaging

50 LENSES AND GEOMETRICAL OPTICS

the front focal point of the objective, while the intermediate image is located at a distance
10–100 times the focal length of the objective in the eyepiece. For more detailed discus-
sions on the topic, refer to Pluta (1988) or Hecht (1998).
Modern microscopes with infinity focus objective lenses follow the same optical
principles already described for generating a magnified real image, only the optical
design is somewhat different. For an objective with infinity focus design, the specimen
is located at the focus of the lens, and parallel bundles of rays emerging from the back
aperture of the lens are focused to infinity and do not form an image; it is the job of the
tube lens in the microscope body to receive the rays and form the real intermediate
image at the eyepiece. The advantage of this design is that it allows greater flexibility for
microscope design while preserving the image contrast and resolution provided by the
objective. Items such as waveplates, compensators, DIC prisms, reflectors, and fluores-
cence filter sets can be placed anywhere in the “infinity space” between the back of the
objective and the tube lens. As long as these items have plane-parallel elements, their
location in the infinity space region of the imaging path is not critical. If we consider the
combination of objective plus tube lens as the effective objective lens, then the same
optical rules pertain for generating a real magnified image and we observe that the rela-
tionship 2f a f is still valid. Sketches showing the infinity space region and tube
lens in upright and inverted microscopes are shown in Color Plates 4-1 and 4-2.
The function of the eyepiece or ocular is to magnify the primary image another
10-fold, and together with the lens components of the eye, to produce a real magnified
image of the intermediate image on the retina. Thus, the object of the eyepiece is the
intermediate image made by the objective lens. Note that in the case of the ocular, 0
a f, so the object distance is less than one focal length, resulting in a virtual image that
cannot be focused on a screen or recorded on film with a camera. However, when the
eye is placed behind the eyepiece to examine the image, the ocular-eye combination
produces a real secondary image on the retina, which the brain perceives as a magnified
virtual image located about 25 cm in front of the eye. The visual perception of virtual
images is common in optical systems. For example, we also “see” virtual images when
we employ a handheld magnifying glass to inspect small objects or when we look into
a mirror.

THE PRINCIPAL ABERRATIONS OF LENSES

Simple lenses of the type already discussed have spherical surfaces, but a spherical lens
is associated with many intrinsic optical faults called aberrations that distort the image
in various ways. Of these faults, the major aberrations are chromatic aberration, spheri-
cal aberration, coma, astigmatism, curvature of field, and distortion (Fig. 4-8). Correc-
tive measures include use of compound lens designs, use of glass elements with
different refractive indexes and color dispersion, incorporation of aspherical lens curva-
tures, and other methods. The tube lens performs an additional important function in
removing residual aberrations of the objective lens. In some microscopes the eyepieces
also help perform this function. Objective lenses are designed to correct for aberrations,
but can never completely remove them. It is common that a solution for correcting one
fault worsens other faults, so the lens designer must prioritize goals for optical perform-
ance and then work toward the best compromise in correcting other aberrations. For
these reasons, objective lenses vary considerably in their design, optical performance,
and cost.