Microscope Objective Parameters and Impacts on Micrograph Quality
Spherical Aberration
A monochromatic conical beam emitted from an axial point of the optical system passes through the optical train. Rays at different aperture angles cannot converge at the same point on the optical axis, forming a blurred spot (commonly known as a blur circle) on the ideal image plane perpendicular to the optical axis. This imaging error of the optical system is called spherical aberration.
Coma
For a monochromatic conical beam emitted from an off-axis object point, after refraction by the optical system, it cannot form a sharp point on the ideal image plane. Instead, it forms a comet-shaped spot with a bright tail. This imaging error is called coma.
Astigmatism
When an oblique monochromatic conical beam from an off-axis object point travels through the optical system, it fails to converge into a sharp image point and only produces a blurred spot. This imaging error is known as astigmatism.
Field Curvature
The sharp image of a flat plane perpendicular to the optical axis formed by the optical system does not lie on an image plane perpendicular to the optical axis, but on a curved surface symmetrical about the optical axis, i.e. the image surface is curved. This optical aberration is field curvature. When focused on the sharp image at the centre of the frame, the periphery of the image becomes blurred; when focused on the sharp image at the periphery, the central part of the image turns blurred.
Chromatic Aberration
When a beam of white light from an object passes through the optical system, light of different wavelengths fails to converge at a single point and forms a coloured spot. This is chromatic aberration. It arises because the refractive index of the same optical glass differs for light of different wavelengths: the refractive index is higher for short wavelengths and lower for long wavelengths.
Distortion
A straight line in the object plane that passes through the optical axis appears curved after imaging by the optical system. This imaging error is distortion. Distortion only affects the geometric shape of the image and does not reduce image sharpness. This is the fundamental distinction between distortion and spherical aberration, coma, astigmatism and field curvature.
When evaluating the quality of a microscope objective, several practical parameters are generally adopted: resolution, clarity and depth of field.
