Page 69 - Fundamentals of Light Microscopy and Electronic Imaging

52 LENSES AND GEOMETRICAL OPTICS

having different color-dispersing properties. For example, glass types known as crown
and flint are paired together to make an achromatic doublet lens that focuses blue and
red wavelengths in the same image plane.
Spherical aberration is the undesirable consequence of having lenses figured with
spherical surfaces, the only practical approach for lens manufacture. Parallel rays inci-
dent at central and peripheral locations on the lens are focused at different axial loca-
tions, so there is not a well-defined image plane and a point source of light at best focus
appears as a spot surrounded by a bright halo or series of rings. For an extended object,
the entire image is blurred, especially at the periphery. One common solution is to use a
combination of positive and negative lenses of different thicknesses in a compound lens
design. Lenses corrected for spherical aberration are intended for use under a specific
set of working conditions. These include the coverslip thickness, the assumption that the
focal plane is at or near the coverslip surface, the refractive index of the medium
between the lens and coverslip, the wavelength of illumination, and others. Thus, users
employing well-corrected lenses can unknowingly induce spherical aberration by using
coverslips having the wrong thickness or refractive index. Special lenses are available
with adjustable correction collars so that spherical aberration can be minimized for
specimens distant from the coverslip, or when it is desirable to be able to use various
immersion media (Brenner, 1994).
Coma refers to a streak of light with the shape of a comet’s tail that appears to
emanate from a focused spot at the periphery of an image. Coma occurs for object points
that are off the optic axis—that is, when object rays hit the lens obliquely. It is the most
prominent off-axis aberration. Rays passing through the edge of the lens are focused
closer to the optic axis than are rays that pass through the center of the lens, causing a
point object to look like a comet with the tail extending toward the periphery of the field.
Coma is greater for lenses with wider apertures. Correction for this aberration is made
to accommodate the diameter of the object field for a given lens.
Astigmatism, like coma, is an off-axis aberration. Rays from an object point pass-
ing through the horizontal and vertical diameters of a lens are focused as a short streak
at two different focal planes. The streaks appear as ellipses drawn out in horizontal and
vertical directions at either side of best focus, where the focused image of a point
appears as an extended circular patch. Off-axis astigmatism increases with increasing
displacement of the object from the optic axis. Astigmatism is also caused by asymmet-
ric lens curvature due to mistakes in manufacture or improper mounting of a lens in its
barrel.
Curvature of field is another serious off-axis aberration. Field curvature indicates
that the image plane is not flat, but has the shape of a concave spherical surface as seen
from the objective. Different zones of the image can be brought into focus, but the whole
image cannot be focused simultaneously on a flat surface as would be required for pho-
tography. Field curvature is corrected by the design of the objective and additionally by
the tube or relay lens and sometimes the oculars.
Distortion is an aberration that causes the focus position of the object image to shift
laterally in the image plane with increasing displacement of the object from the optic
axis. The consequence of distortion is a nonlinear magnification in the image from the
center to the periphery of the field. Depending on whether the gradient in magnification
is increasing or decreasing, the aberration is termed pincushion or barrel distortion after
the distorted appearance of a specimen with straight lines such as a grid or reticule with
a pattern of squares or rectangles. Corrections are made as described for field curvature.