Which type of aberration results in uneven focus across a lens?

The type of aberration that results in uneven focus across a lens is spherical aberration. This phenomenon occurs when light rays that pass through different parts of a spherical lens do not converge at the same point after refraction. Instead, rays that pass near the edge of the lens focus at a different point than those that pass through the center, leading to a blurred image or uneven focus across the image plane.

Spherical aberration is significant because it impacts the quality of the image produced, causing distortion and a lack of sharpness. This effect is particularly noticeable in lenses that are purely spherical, as opposed to ideal lenses, which would focus all rays at a single point.

In contrast, while other types of aberrations can affect image quality, they do so in different ways. For example, transverse and longitudinal aberrations deal with the spread of light rays in the image plane but are defined in contexts related to magnification and depth of focus. Astigmatism results from a lens or optical system not having the same curvature in all directions, which leads to different focal points for different orientations. Thus, spherical aberration specifically pertains to the uneven focus across a lens due to its geometric shape.