In optics, Aberration is an optical system's deviation from true, such as points in the image being smeared and/or shapes being changed. Lenses and mirrors inevitably cause some aberration which can be the primary limit to a telescope's use. They come from a variety of causes resulting in a variety of symptoms. Some classes:

• chromatic
• spherical
• Coma
• astigmatism
• field curvature
• pincushion
• barrel
• distortions
• Focal Plane tilt