A natural fog causes lights to glow and flare. Contrast is lower, and sharpness may be affected as well. Fog filters mimic the effect of atomized water droplets in the air. For example, the effect of humidity in a tropical scene can be created or enhanced. In their lower grades, these filters can take the edge off excess contrast and sharpness. Higher grades can create unnatural effects, such as in fantasy sequences.
Tiffen Fog filters are generally available in grades 1/8 through 5, with 5 having the strongest effect. In general, however, the effect of a strong natural fog is not produced accurately by Fog filters in their stronger grades. For that, Double Fog filters are recommended.