Chromatic aberrations occur when the refractive index of a material depends on the wavelength (dispersion) so that a lens focuses different colours to different focal points. For materials with normal dispersion, the refractive index decreases with increasing wavelength, so that longer wavelengths have longer focal lengths. Chromatic aberrations can be reduced by increasing the focal length of the lens or by using an achromatic lens (achromat), where materials with differing dispersion are assembled together to form a compound lens. Also, many types of glass have been developed to reduce chromatic aberrations, such as glasses containing fluorite, which are often employed in the realisation of commercial microscope objectives [Subsection 8.2.1].
Spherical aberrations occur when the light rays that strike a lens near its edge are deflected differently than those that strike the lens nearer the centre. A positive (negative) spherical aberration occurs when peripheral rays are bent too much (are not bent enough).
The coma, or cometic aberration, is a variation in magnification over the entrance pupil, so that the image of an off-axis object is flared like a comet, hence the name.
The astigmatism is due to the fact that rays that propagate in two perpendicular planes have different foci, which are in fact line foci, inclined in orthogonal directions and separated by some axial distance.
The Petzval field curvature, or field distortion, is the optical aberration in which a flat object normal to the optical axis cannot be brought into focus on a flat image plane.
Fig. 2.B2 — Optical aberrations