Section Overview:
An ideal microscope objective produces a symmetrical diffraction limited image of an Airy pattern from an infinitely small object point. The image plane is generally located at a fixed distance from the objective front lens in a medium of defined refractive index. Microscope objectives offered by the leading manufacturers have remarkably low degrees of aberrations and other imperfections, provided the appropriate objective is selected for the task and the objective is utilized properly in accordance with the manufacturer's recommendations. It should be emphasized that objective lenses are not made to be perfect from every standpoint, but are designed to meet certain specifications depending on their intended use, constraints on physical dimensions, and price ranges.
Objectives are made with differing degrees of optical correction for both monochromatic (spherical, astigmatism, coma, distortion) and polychromatic aberrations, field size and flatness, transmission wavelengths, freedom from fluorescence, birefringence and other factors contributing to background noise. Depending upon the degree of correction, objectives are generally classified as achromats, fluorites, and apochromats, with a plan designation added to lenses with low curvature of field and distortion. This section addresses some of the more common optical aberrations that are commonly found (and often corrected) in microscope objectives.
Review Articles
Overview of Optical Aberrations
Departures in lens action from the ideal conditions of optics are known as aberrations. Optical trains typically suffer from as many as five common aberrations: spherical, chromatic, curvature of field, comatic, and astigmatic.
Field Curvature
Curvature of field in optical microscopy is an aberration that is familiar to most experienced microscopists. This artifact is the natural result of using lenses that have curved surfaces.
Spherical Aberration from Coverslip Thickness Variations
For microscope objectives having apertures, the optical properties and thickness of the medium lying between the front lens element and the specimen affect the calculations necessary to correct for image aberrations.
Interactive Tutorials
Astigmatism
Explore the relationship between astigmatism and comatic aberrations, and how astigmatism aberrations are manifested by the off-axis image of a specimen point appearing as a line or ellipse instead of a point.
Chromatic Aberration
Learn more about chromatic aberrations and how when white light passes through a simple or complex lens system, the component wavelengths are refracted according to their frequency.
Comatic Aberration
Learn about comatic aberrations and how they are mainly encountered with off-axis light fluxes and are most severe when the microscope is out of alignment, as well as the result of when these aberrations occur.
Curvature of Field
Curvature of field in optical microscopy and how it is a common and annoying aberration that is familiar to most experienced microscopists is explained in this featured interactive tutorial.
Geometrical Distortion
Explore the two most prevalent types of distortion, positive and negative, and how they can often be present in very sharp images that are otherwise corrected for spherical, chromatic, comatic, and astigmatic aberrations.
Spherical Aberration
Learn about the most serious of the monochromatic defects that occurs with microscope objectives, the spherical aberration, which causes the specimen image to appear hazy or blurred and slightly out of focus.
Focus Depth and Spherical Aberration
Discover an aberration-free meridional section of a point source of light located at a depth in the specimen layer having a refractive index and imaged with a virtual microscope objective.
Coverslip Thickness Correction
A demonstration on how internal lens elements in a high numerical aperture dry objective may be correctly adjusted with these varied cover glass thickness and dispersion fluctuations is featured in this tutorial.
Selected Literature References
Selected Literature References
Aberrations are divided into two main categories: errors that occur when polychromatic light (white light) is passed through a lens, and errors that are present when only a single wavelength (monochromatic) of light is utilized. The selected references listed in this section contain information about the cause and correction of the most common optical aberrations encountered with microscope and other lens systems. Bear in mind that the optical designer must correct for both polychromatic and monochromatic aberrations simultaneously in the production of well-corrected microscope objectives.