Spherical aberration is also referred to as aperture error. This error occurs on (normal) spherical lenses with a fixed radius.
They exhibit different focal lengths in the centre and at the edge, rays of light which pass through the lens edge have a longer focal length than centric rays and are focused before or behind the sensor. Lenses with significant spherical aberration produce a soft and slightly blurred image, as the sharp core image interferes with a blurred one.
Remedy is provided by stopping down the lens or using aspheric lens elements. However, their production is clearly more expensive due to different radii of curvature on the glass surface, they must be processed individually on CNC machines in most of the cases.
Important for machine vision
- Spherical aberration can slightly be compensated by stopping down.
- Buy proper optics the lens design of which corrects as many errors as possible and/or uses achromatic lenses.