DIY Soft Focus
Unless you're a portrait photographer using a large format camera, you may not have ever heard of the Rodenstock Imagon lens. Although no longer made, this commercial soft-focus lens once had quite a group of loyal users.
The lens acheived its effect through a simple method of controlling spherical aberration.
Anyone who has built or used simple camera lenses, such as those made from a magnifying glass or a single convex element, has seen the soft look of spherical aberration. Spherical aberration results from the fact that light passing through the center of an uncorrected lens comes to focus at a different point than light passing though the outer part of the lens. This results in widening the depth of field, reducing the contrast, and adding a glow around highlights. The amount of spherical aberration is dependent on aperture. Stopping down the lens reduces the effect.