The 100mm (4") Master Neutral Density Filter Kit 4 from Hitech contains six filters; three Solid Neutral Density Filters (0.3, 0.6, 0.9) and three Graduated Neutral Density Filters (0.3, 0.6, 0.9). With this kit, the outdoor photographer can create some wonderful images and is also prepared to shoot under a wide variety of lighting conditions. Inasmuch as the Master Kit is complete with both solid and grads, there will be no need to buy additional ND filters. The basic advantage is that it enables the shooter to adjust exposure without affecting color balance. An ND filter appears gray and reduces the amount of light reaching either the sensor of a digital camera or the film plane of a traditional film camera.
A Solid Neutral Density filter is used to create some unusual special effects such as capturing the "blur" of the rippling water of a waterfall or the swirling effect of city traffic. Since it allows the use of slower shutter speeds in bright light, it becomes possible to create these special effects that would normally not be possible to capture without the filter. And, since it can also help to control your depth of field, it permits you to shoot at wider apertures. Ultimately, its value is in helping to prevent bright, overly washed out images shot in bright light. Soft/Hard designation indicates the degree of transition of the filter.
A Graduated Neutral Density filter is used to balance the light intensity in one part of a scene with another. This is especially true in situations where you don't have total light control, as in bright contrasty landscapes. Exposing for the foreground will produce a washed-out, over-exposed sky while exposing for the sky will leave the foreground dark and under-exposed. Hitech Grad ND filters are part clear, part neutral density. They allow the transition to be blended into the scene, often imperceptibly.
Soft edge grads have a smooth transition from clear to ND. Hard edge grads have a more defined hard lined transition and should be used for longer lenses.