Neither soft focus filters on the camera nor diffusion in the darkroom can reproduce the effect of a soft-focus lens; for one thing (of many...) the filters cannot adjust the amount of diffusion in relation to the depth of field. In other words, an image that was originally sharp in the background will, in the filter-softened version, have a background that is diffused to the same extent as the foreground. Soft-focus lenses, on the other hand, are normally used at large aperture, and the softness varies quite strongly with distance from the plane of best focus. Furthermore, amount of aberration typically increases with radial distance from the optical center, an effect that is not quite reproduced by even gradient-diffusion filters.