Aberration, in optics, a condition that causes blurring, loss of clearness, or distortion of shape in the images formed by lenses or curved mirrors. Although it is impossible to produce a telescope, microscope, camera lens, or other optical device that is free from aberrations, optical engineers can eliminate or reduce most of these effects by careful design. The two most familiar types of aberration are chromatic aberration and spherical aberration.

Chromatic aberration occurs when a ray of light containing more than one color (for example, a ray of white light) is spread out slightly by refraction when it passes through a lens. Refraction. The resulting image is surrounded by blurred, colored fringes. Chromatic aberration does not occur in images formed by mirrors. The absence of this type of aberration is one of the most important advantages of the reflecting telescope over the refracting.

Spherical aberration occurs when light rays striking near the edges of a lens or curved mirror are brought to a focus closer to the lens or mirror than are the central rays. The result is that the image of a point appears as a small disk.

Other aberrations, known as coma, astigmatism, curvature of field, and distortion, can also occur when the source of the image, or any part of the source, is not on the optical axis of the lens or mirror.