Page 68 - Fundamentals of Light Microscopy and Electronic Imaging

THE PRINCIPAL ABERRATIONS OF LENSES 51

On-axis aberrations

Blue Green Red

Chromatic Spherical
(a) (b)

Off-axis aberrations

Astigmatism
Coma
(c) (d)

Distortion

Field curvature Barrel distortion Pincushion distortion
(e)

Figure 4-8
Aberrations of a simple lens. (a) Chromatic aberration: Parallel incident rays of different
wavelength are focused at different locations. (b) Spherical aberration: Incident rays parallel
to the optic axis and reaching the center and the periphery of the lens are focused at
different locations. (c) Coma: Off-axis rays passing through the center and periphery of the
lens are focused at different locations. (d) Astigmatism: An off-axis aberration causes waves
passing through the vertical and horizontal diameters to focus an object point as a streak.
(e) Distortion and field curvature: The image plane is curved and not planar. So-called barrel
and pincushion distortions produce images that are not high in fidelity compared to the
object.

Chromatic aberration occurs because a lens refracts light differently depending on
the wavelength. Blue light is bent inward toward the optic axis more than red light. The
result is disastrous: Blue wavelengths are focused in an image plane closer to the lens
than the image plane for red wavelengths. Even at the best focus, point sources are sur-
rounded by color halos, the color changing depending on the focus of the objective, the
image never becoming sharp. Since each wavelength is focused at a different distance
from the lens, there is also a difference in magnification for different colors (chromatic
magnification difference). The solution is to make compound lenses made of glasses