A spherical aberration is a visual effect caused by the distortion of light as it passes through a spherical lens. The effect degrades the contrast of an object being observed through the lens and causes a single point to be surrounded by a halo of light. It will create a blurring of detail of the entire image, and any contrast between an object’s features will be poorly defined. Spherical aberration occurs uniformly throughout the field of the lens, regardless of how much or how little light passes through.
Named for the point of generation at the spherical surface of the lens, spherical aberration is common for optical lenses, especially those found in telescopes. Changes in the distance of objects can trigger the effect. The waves of light that pass through it do not meet up on the mirror, preventing a clear image from forming. It is important not to over-correct the aberration, in which case the effect would appear the same. When under-corrected, the change in rays causes the image to be reversed.
To design a lens or correct an aberration error, a spherical aberration coefficient is calculated, which is equivalent to the wavefront error that creates the effect. For under-corrected spherical aberrations this coefficient is negative in value, and aberrations that are over-corrected have a positive value. Correcting the problem involves being able to balance the spherical aberration by adjusting the waves of light in the appropriate direction.
Spherical aberration can be corrected through one of a variety of measures. Decreasing the size of the lens aperture is often enough to fix the problem. The amount of blur lowers exponentially with the diameter of the aperture, which is important to correct this visual error. This calculation is different if compound lenses are being used together in a system. Special filters are sometimes used by photographers in their cameras to create a spherical aberration and produce special effects on the resulting images.
When it is necessary to fix spherical aberrations, the proper calculations must be taken into account. These mathematical elements include coefficients and variables that must be specific to the lens and error that is taking place. The problem can be corrected and the focus adjusted so that the image is clear, a requirement of all cameras, microscopes, as well as both amateur and the most advanced telescopes.
By Andrew Kirmayer