Optical aberration

Lore & Background

In optics, aberration occurs when light from one point of an object does not converge into (or does not diverge from) a single point after transmission through the system. Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements. An image-forming optical system with aberration will produce an image which is not sharp, and makers of optical instruments need to correct optical systems to compensate for aberration.

Reader's Guide

Optical aberration is a fundamental concept in optics, describing the failure of lenses and mirrors to produce perfect images. It is divided into two classes: monochromatic aberrations, caused by the geometry of the lens or mirror and occurring even with monochromatic light, and chromatic aberrations, caused by dispersion (the variation of a lens's refractive index with wavelength). The most common monochromatic aberrations include defocus, spherical aberration, coma, astigmatism, field curvature, and image distortion. Chromatic aberrations are axial (longitudinal) and lateral (transverse). The Gaussian theory of optics provides a convenient approximation but is only true for infinitely small angles; realistic systems always exhibit aberrations. The sine condition, as noted by Abbe, is required to avoid certain aberrations for points off the axis, and a system fulfilling this condition and free from spherical aberration is called aplanatic. Aberration increases with lens diameter and aperture, and can be minimized by reducing the aperture, though this also reduces light gathering.

Did You Know?

• Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements.
• Chromatic aberration does not appear when monochromatic light is used.
• Piston and tilt are not true optical aberrations, since when an otherwise perfect wavefront is altered by piston and tilt, it will still form a perfect, aberration-free image, only shifted to a different position.
• The word 'aplanatic' was first used by Robert Blair to characterize a superior achromatism, and subsequently by many writers to denote freedom from spherical aberration as well.