Soft Rings Diffusion Filters are a series of filters with concentric rings that are cast into the glass. As the diffusion increases, the contrast level will remain virtually unchanged. Photo Control recommends that these filters be used in the normal portrait range of apertures such as f/8-f5.6. Excessive depth-of-field could produce an uneven diffusion effect. Soft rings provide an ideal diffusion in conjunction with all Lindahl Vignettes.