Understanding Optical Aberrations

What is an Optical Aberration?

Imaging systems work by focusing light that passes through a lens (or several lenses) and falls on a sensor or a screen. Lenses are bidirectional so this applies just as much to projector systems as to cameras and other imaging systems.

Optical System

In poorly-designed optical systems it is common to see distortion or poor focus in some or all regions of the image. This is known as an optical aberration. These optical aberrations take many forms. Correction is best addressed during the design process when optical components are being selected. Although in some cases it is possible to correct the distortion or focus after the fact.

In this article we discuss the causes of optical aberrations, the most common types, and offer some ways to avoid them.

Causes of Optical Distortion

Lens designers use this behavior to focus light at a focal point and create a mirror image on a sensor. Alternatively, a lens may gather light spreading out from the focal point and disperse it over a wide area. The first setting is inherent in cameras, the second in projectors.

However, lenses don’t exactly perform as predicted by mathematical or theoretical models. In addition, wavelength is a factor in refraction. And, while lenses pass a circle of light, most sensors and screens feature a landscape format. As a result, optical aberrations are more pronounced at the left and right edges of images as these are the farthest from the optical centerline. Designers compensate for these aberrations by adding additional elements or optical coatings. However, this makes the system bigger, heavier and more expensive. Every lens is therefore a compromise between size, weight and cost and the quality of the resulting image.

Chromatic vs. Monochromatic Aberration

A desirable form of chromatic aberration occurs when a prism splits white light into a rainbow. In a camera or projector system the result is less attractive. Here chromatic aberration appears as images of the same object but at different magnifications. Edges show a degree of blurring with colors spanning the rainbow, blue towards the center and red outermost.

The extent to which light changes direction at the interface between media of different optical densities is a function of wavelength. Blue light, (wavelength 470nm) is diverted more than red light (wavelength 660nm.) An optical lens intended for use with a white light illumination system must take this into account or the image will exhibit chromatic aberration.

Optical Aberration — Chromatic

Chromatic aberration appears as images of the same object but at different magnifications. Edges show a degree of blurring with colors spanning the rainbow, blue towards the center and red outermost.

Where possible, the best way to avoid this problem is by using monochromatic light. Chromatic and monochromatic systems can still suffer from other types of aberration, but monochromatic light eliminates this type of distortion.

Alternatively, applying optical coatings to the surfaces of the lenses can reduce the magnitude of the effect.

Types of Optical Aberrations

Spherical

Optical Aberration: Spherical

Coma

Optical Aberration: Coma

Astigmatism

Optical Aberration: Astigmatism

Distortion

Optical Aberration: Pincushion

Barrel is the opposite where the sides stretch out along the vertical and horizontal and vertical axes so the image has the shape of a barrel. Barrel distortion is common with wide angle lenses where light is refracted through a large angle. This is exhibited in fisheye lenses and is also seen in a hemispherical mirror. The solution is to use a lens with a longer focal length. Pincushion is the opposite, occurring at long focal lengths.

Optical Aberration: Barrel

Field Curvature

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