
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.