Spherical Aberration
The soft images result from the fact that simple, uncorrected lenses do not focus light perfectly. Instead, they have an array of aberrations, the dominant one typically being Spherical Aberration (SA).
SA results from the fact that parallel rays of light passing through the center of an uncorrected lens come to focus at a different point than the light rays passing though the outer part of the lens. The light passing through the lens near the center comes to focus as expected at the focal length. However, light passing through the lens toward the edge focuses at a shorter distance.
This results in widening the depth of field, reducing the contrast, and adding a glow around highlights.