The Tiffen 138mm Softnet Black 4 Filter is a softening filter meant to replicate the look of draping silk over the lens in order to soften or diffuse light. This filter works by scattering light entering the lens through a mesh pattern, resulting in an overall even, soft glow. The black distinction helps to maintain the overall contrast of the scene by reducing the overall spread of light into the shadows. This filter has a 4 density, providing the greatest amount of softening.
Tiffen filters are made using ColorCore technology, a process that involves laminating the filter substrate between 2 pieces of optical glass, grinding flat to a tolerance of 1/10,000th of an inch, and then mounting to precision metal rings.