Spherical Aberration is an optical problem that occurs 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 optical 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 the bottom of the illustration end up converging at a shorter distance along the optical axis (black/red dotted line), while the ones closer to the optical axis converge at a longer distance, creating different focal points along the same axis. The point of best focus with the “circle of least confusion” is illustrated as the thick green line. Spherical Aberration is not just caused by lens design, but also by the quality of the lens material. Lenses made of poor quality material and large bubbles can drastically impact light refraction.