Neutral Density filters evenly reduce the amount of light passing thru the lens without effecting color. Ideal for reducing bright light in large aperture situations when a narrow depth is field is needed. Allows for slower shutter speeds when blurring or showing of movement is desired. Allows balancing of exposure to highlight a key subject. Lets you use higher speed films in bright light. B&W Neutral Density filters are available in the following strengths: .3 (2X) #101 Reduces the light one f-stop. .6 (4X) #102 Reduces the light two f-stops. .9 (8X) #103 Reduces the light three f-stops. 1.8 (64X) #106 Reduces the light six f-stops 3.0 (1,000X) #110 (ten f-stops) & 4.0 (10,000X) #113 (seventeen f-stops) are for astrological and sun studies