Neutral Density filters, otherwise known as ND filters, are designed to cut down the amount of light that comes into a scene and therefore allow you to either use a more narrow aperture or a slower shutter speed. They’re most commonly use with landscape photographers, but are also known to be popular with strobists that want to overpower the ambient light in their scenes.
Breakthrough Photography has been manufacturing ND filters for a little while now, and in many ways, they’re an incredibly innovative and logical company that creates products that simply work. The company’s 6 stop ND filter is simply just that–an ND filter that cuts down up to six stops of light. Though vari-ND filters are much more valuable and useful, lots of shoots prefer a filter that was designed specifically to do an assigned job.