A friend and former student has a small old aplanat/rapid rectilinear (two cemented doublets) that's been in the family for several generations, and would like to use it as a soft focus lens on a 4x5. It's rather sharp, and she doesn't want to use a diffusing filter. We're looking for a way to induce actual spherical aberration into the design.

Ernest PurDum's excellent article on soft focus lenses on the lf homepage has the following passage:

"There are ways of upsetting carefully calculated spherical aberration reduction besides changing element spacing. A thick glass plate is one. Adding various combinations of weak positive and negative elements, Proxar and Distar and the like, might be another."

Any thoughts on placing a quarter- or half-inch piece of glass inside the lense barrel to create aberrations? Other thoughts on how we might get the desired look out of it without causing permanent damage?