Aberrations: Key Factors Affecting Microscope Imaging
① Chromatic Aberration Chromatic aberration is a serious defect in lens imaging and occurs with polychromatic light sources; monochromatic light does not produce chromatic aberration. White light consists of seven colors: red, orange, yellow, green, cyan, blue and violet. Since each color has a different wavelength, the refractive index varies when passing through a lens. Consequently, a single point on the object side may form a colored spot on the image side. The primary function of an optical system is to correct chromatic aberration.
Chromatic aberration generally includes longitudinal chromatic aberration and lateral chromatic aberration. Longitudinal chromatic aberration causes colored spots or halos on the image regardless of the observation position, blurring the image. Lateral chromatic aberration produces colored fringes around the image.
② Spherical Aberration Spherical aberration is a monochromatic aberration for on‑axis points, caused by the spherical surface of a lens. It results in a bright spot with a bright center and gradually blurred edges instead of a sharp point image, which degrades image quality.
Spherical aberration is usually eliminated by lens combinations. Convex and concave lenses have opposite spherical aberrations, so cemented doublets made of different optical materials can be used for correction. Older microscope objectives with incompletely corrected spherical aberration must be used with matching compensating eyepieces. Spherical aberration of objectives in most modern microscopes is fully
corrected within the objective itself.
③ Coma Coma is a monochromatic aberration for off‑axis points. When an off‑axis object point is imaged by a large‑aperture light beam, the light rays no longer converge to a single point after passing through the lens. The image of a light spot becomes a comet‑shaped flare, hence the name "coma".
④ Astigmatism Astigmatism is another off‑axis monochromatic aberration that reduces sharpness. For a large field of view, object points at the edge are far from the optical axis with highly inclined light beams, which induce astigmatism after passing through the lens. Astigmatism converts an original object point into two separate, mutually perpendicular short line segments, which combine into an elliptical spot on the ideal image plane. Astigmatism is corrected by sophisticated lens assemblies.
⑤ Field Curvature Field curvature is also known as "Petzval field curvature". When field curvature exists, the focal points of the entire beam do not coincide with the ideal image plane. Although sharp images can be obtained at individual positions, the overall image surface is curved. This makes it impossible to view the whole field sharply at the same time, bringing difficulties for observation and photography. For this reason, research microscopes usually employ plan objectives, in which field curvature has already been corrected.
⑥ Distortion Except for field curvature, the aberrations described above affect image sharpness. Distortion is a different type of aberration. The concentricity of light beams remains intact, so sharpness is not impaired. However, distortion causes geometric deformation of the image compared with the original object.
