An aberration prevents light from being brought into sharp focus. The ideal image by lenses (especially photographic lenses), must fulfill three key conditions, namely, 1. all light from the point object must be focused to a single point on the image plane (film or digital sensor); 2. when the object plane is perpendicular to the optical axis, the image plane must also be perpendicular; and 3. the object and the image (on the film or imaging sensor) must closely resemble each other. In reality, however, light refraction by the lens causes a variety of defects in the image, called aberration. The five most common types in aberration are SEIDEL's five aberrations, which occur even with monochromatic (single-wavelength) light. In addition, there are also two types of chromatic aberrations, which are caused by more than one frequency. It is impossible to eliminate them all, but in the lens design process, they can be controlled and balanced to provide the best possible result for that lens and application. This is handled by material (glass) selection, shape and positioning.