The term optical aberration sums up all deviations in the image forming properties of lenses and optical systems. These deviations can be differentiated into chromatic aberrations and monochromatic aberrations, the later being subdivided into astigmatism, coma, distortion, field curvature and spherical aberrations.
Chromatic aberration
The term chromatic aberration being derived from the Greek chroma (colour) and the Latin ab-errare (off-err) describes the fact that simple collecting lenses have different focal length for blue, green and red light. Because light of different wavelength is focused to different points, objects displayed by those lenses, show spurious coloured halos. Thus compound lenses were designed, made of crown and flint glass. These materials disperse the continuous spectrum differently and when combined like in achromatic objectives the chromatic aberrations for blue and red wavelengths just level out (red-blue corrected).
Spherical Aberration
Common lenses have spherical surfaces due to simplicity of production. An unwanted effect is that rays parallel to the optical axis have different focal points depending on the distance from the axis. Lens producers correct spherical aberrations by using compound lens designs or more expensive aspherical lenses. The correction usually is not universal but is assumed to work under certain conditions, defined by the coverglass thickness, wavelength of light, refractive index of medium between lens and coverglass, etc. Objectives with correction collars allow the user to adapt the spherical aberration correction to his changing working conditions (e.g. specimen with variable thickness, use of different immersion media) if necessary.
Astigmatism
Astigmatism is an off-axis lens aberration. Specimen points positioned at outer regions of the field of view away from the optical axis are focused as streaks by uncorrected lenses while the image points projected to the image plane are blurred (circle of least confusion). Reasons for astigmatism are asymmetric lens curvatures due to skewed lens mounting or low quality manufacture.
Coma
Another off-axis aberration is called coma. Rays coming from specimen points at the periphery of the field of view and passing either through the periphery or the centre of the lens are focused asymmetrically at different locations of the image plane.
Distortion
Distortion is caused by changing magnification factors for object details with increasing distance from the optical axis. If the magnification increases with distance, it produces pincushion distortion; if it decreases the effect is called barrel distortion.
Curvature of field
Simple spherical lenses project the specimen details onto a curved image plane. Thus in the field of view only parts of the image are in precise focus. The user will have to refocus continuously to get a sharp impression of all details. Because the human eye can do part of this refocusing job, slight deviations from planarity are not recognised immediately, but can cause fatigue in long term use. Cameras do not re-focus automatically and the resulting images are not in focus over the entire field of view.
Curvature-of-field aberrations are corrected by the objective lens design and sometimes by additional tube/relay lenses inside the microscope frame or even by the oculars. When microscopists exchange objectives and oculars from different suppliers the plan correction may be lost (or overcorrected), if these manufacturers apply different strategies to prevent curvature of field.
