Solid Neutral Density (ND) Filters
Solid neutral density filters reduce the amount of light entering your lens. The primary purpose of these filters, which are rated by how many f-stops (EV) of light they block, is to allow photographers to use slow shutter speeds in bright conditions. Solid-ND filters are thus useful to anyone looking to use a slow shutter speed for creative or technical reasons. For example, you might want to shoot a portrait using a wide aperture, but in bright light your camera might not be able to use a fast enough shutter speed to properly expose the scene (most shutters top out at around 1/4000 to 1/8000s).
Solid-ND filters are useful for videographers, who often try to capture video with a shutter speed of 1/30s to minimize artifacts. My favorite use for solid-ND filters is to capture extremely long exposures to blur motion and smooth water, for a creative effect you just can’t get normally in bright conditions.
The downside of solid-ND filters is that poorly constructed ones will be extremely prone to color casts and can reduce image sharpness. I use Singh-Ray solid ND filters as they are some of the highest quality glass filters available.
Verdict: If you enjoy shooting with wide apertures or want to capture creative long exposure images, solid-ND filters are a must-have.