The visible area which appears in focus is the flat field. As magnification power increases, the working distance decreases. This is a distortion caused by the lens. Spherical aberration is distortion of the image, as explained in Flatness of Field. Chromatic aberration is distortion of colors being viewed, as explained in Color Distortion.
Lenses produce a prism effect which causes the image to develop false color fringes.
This is due to the fact that different colors focus at different points. Achromatic lenses correct this problem by focusing many colors at the same point. Parallax is the apparent misalignment of two different items when viewed from an angle. It is the cause of improper registration when stripping multiple color jobs. Parallax is eliminated when viewing straight down on registration marks instead of from a slight angle.