Spherical aberration is an optical lens fault that occurs when light rays passing through the outer edges of a lens focus at a slightly different point than rays passing through the centre. This misalignment prevents all light from converging accurately on the same image plane, resulting in a loss of sharpness and overall image definition. The effect can make photographs appear soft or slightly blurred, particularly around the edges of the frame.
Spherical aberration is most noticeable when shooting with a lens at its widest aperture, as more of the outer lens elements are being used. Its effects can usually be reduced by stopping the lens down to a smaller aperture, which limits the amount of peripheral light entering the lens and improves sharpness. Modern lens designs often include corrective elements to minimise spherical aberration, but understanding the issue remains important when evaluating lens performance and image quality.