The Tiffen 138mm Warm Black Diffusion/FX 4 Filter diffuses strong light entering the lens and produces a glowing effect. While the resulting image appears softer and ethereal, there is little loss of clarity or detail. This filter works by diffusing the light entering the lens with small aberrations; by spreading the light more evenly, a soft glow is created rather than the sharp, well-defined image when shooting without a filter. In addition to the diffusion provided, a warming effect is also integrated into the filter helping to further balance skin tones. This filter has a 4 density, providing a noticeable 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.