That depends on what you want to do. Basically, ND filters differ mainly in the degree of darkening. We have filters in 3 different strengths: 8x, 64x and 1000x. The number indicates the factor by which the shutter speed is extended. With a 64x filter, for example, your shutter speed will be 64x longer. 1 second exposure time without a filter becomes 64 seconds with a 64x ND filter.