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 most sizes and in the following strengths: .3 (2x) Reduces the light one f-stop. .6 (4x) Reduces the light two f-stops. .9 (8x) Reduces the light three f-stops. 1.8 (64x) Reduces the light six f-stops 3.0 (ten f-stops) & 4.0 (seventeen f-stops) are for astronomical and sun studies