Spherical Aberration in Camera lenses

 

What is Spherical Aberration?

Spherical Aberration is an optical problem that happens when all incoming light rays end up focusing at different points after passing through a spherical surface. Light rays passing through a lens near its horizontal axis are refracted less than rays closer to the edge or “periphery” of the lens and as a result, end up in different spots across the lens axis.

In other words, the parallel light rays of incoming light do not converge at the same point after passing through the lens. Because of this, Spherical Aberration can affect resolution and clarity, making it hard to obtain sharp images.


Here is an illustration that shows Spherical Aberration:

As shown above, light rays refract or change their angle when passing through the lens. The ones closer to the top and bottom of the illustration end up converging at a shorter distance along the lens axis (black/red dotted line), while the ones closer to the lens axis converge at a longer distance, creating different focal points along the same axis.

Spherical Aberration is not just caused by lens design, but also by the quality of the lens material.L enses made of poor quality material and large bubbles can drastically impact light refraction.
A perfect lens would have all light rays converge in a single focal point, as illustrated below:

A normal spherical lens design would not allow the above to happen though, so specialized precise methods by manufacturers have been developed over the years to reduce the effect of spherical aberration.

Ways to Reduce Spherical Aberration

Modern lenses employ different techniques to dramatically reduce spherical aberration. One of the methods employs using a specialized apsherical (meaning non-spherical) lens surface, which is curved outwards on one side for the sole purpose of converging light rays into a single focal point, as illustrated below:

Spherical aberration is most pronounced when the diaphragm of the lens is wide open (maximum aperture). Stopping down the lens even by a single stop dramatically reduces spherical aberration, because aperture blades block the outer edges of spherical lenses.

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