What is spherical aberration?

Spherical aberration refers to an optical phenomenon that occurs when light rays that pass through the edges of a lens do not converge at the same point as those that pass through the center. This results in a curved image instead of a sharp, well-defined one when focusing on a flat object. The underlying reason for this behavior is that spherical lenses have a shape that causes light rays to refract differently depending on their angle of entry. This results in a blurring effect, particularly prominent in images where precise focus across the entire field of view is essential.

In contrast, the other options pertain to different optical issues or effects that do not accurately describe spherical aberration. For example, the inability to focus on edges may relate to other types of aberrations or imperfections in a lens system, double vision is a symptom often associated with misaligned optics or conditions affecting vision rather than the characteristics of spherical aberration, and the assertion that this effect only occurs in telephoto lenses is misleading, as spherical aberration can occur in any spherical lens, regardless of its type or application.