The Freewell Variable Neutral Density (ND) Filter lets you vary the amount of light entering the lens by a range of one stop to more than eight stops.

It will quickly fit the Phantom Pro or Advanced after unscrewing the stock UV filter. To vary the ND density, simply rotate the front ring to dial in the correct exposure.

The ND2-400 filter replaces 10 individual ND filters.

Like any ND filter, it's also useful for when a narrower depth is field is required.

The only real downside of a Variable compared to a full set of individual NDs is the lack of measurability. 

For example, if you previously liked the result from your Phantom with an ND16 filter, you can't just grab a Variable and select ND16. With a Variable, you need to rotate it to find the most suitable position for the shot. Some people don't mind that, others do...

The Freewell Variable ND features:

• HD optical glass with Multilayer coating 
• Easy grip aluminum alloy filter ring construction
• Lightweight (3gm) with no effect on gimbal balance
• 4K Series filters are specifically designed for professional quality 4K video and high resolution still images
• Includes padded metal tin for secure storage

If you want to experiment with NDs without the expense and effort of carrying a full set with you, then this is definitely for you.