Spherical Aberration
Image softness that appears when a lens focuses rays through its edge and its center at slightly different points, blurring fine detail across the field.
Spherical aberration is an optical defect of a simple spherically ground lens, where light passing near the rim comes to focus at a different point than light passing through the center. The result is a soft, slightly hazy image that never quite snaps to a crisp focus, because no single setting brings every ray together at once. It is a different problem from chromatic aberration, which is about color rather than the shape of the focused point.
Designers correct it by combining elements of carefully chosen curves, by using aspheric surfaces, and by pairing crown and flint glass so the errors cancel. A well-built objective lens group keeps spherical aberration low so the image stays sharp at high power, and quality glass such as ED glass often accompanies that correction. A scope that controls these aberrations gives a cleaner picture and lets the shooter resolve smaller detail at long range.