Variable 2-8 stops of variable density
Reduce brightness without affecting color or depth of field
Enable slower shutter speeds to cause blurring effect in water, clouds or cars.
Decrease depth of field by allowing large apertures to be used while keeping shutter speeds lower.
For DSLR Video: Help optimize your shutter speed – keeping it double your FPS rate.
When using film: Decrease the sensitivity of high speed ISO films in bright outdoor conditions.