Over the past year that I have owned my Phantom 3 Professional I have seen hundreds on questions asking what are ND filters for? The short explanation of what ND filters do is that they are sunglasses for your camera. Just as sunglasses reduce the amount of light to your eyes outdoors, a ND filter reduces the amount of light that goes to your camera's sensor. They are nothing more than tinted glass or plastic that goes over the front of your camera. The larger the ND number the, the darker the tint of the filter and the less light let through. For example a ND2 filter would let a lot more light through than a ND64 filter.
ND filters are available in ND2, ND4, ND8, ND16... Each time you move to a higher number ND filter, the tint doubles in darkness. For example ND4 is twice as dark as ND2, and ND8 is four times as dark as ND2. So what would be the purpose of using a ND filter with a Phantom 3/4? The primary use would be for video, but there are a few reasons why one may use them for photography as well. The next part may be confusing if you do not understand photography 101. All cameras have three controls in order to properly expose an image - aperture, ISO, and shutter speed.
A common rule of thumb for recording video on any camera is that you double your shutter speed based on whatever fps (frames per second) you're shooting in. For example if you are shooting 4k and 30fps, your shutter speed should be 1/60 sec (30fps x 2 = 60). The reason for this is to achieve smooth video and avoid any moire or jello effect you may see at higher shutter speeds. Since the Phantom 3 camera is locked at f2.8, you are only able to adjust the ISO or the shutter speed to properly expose your image. Having your ISO at the lowest possible setting (100) and an aperture as wide a f2.8, the only way to reduce the shutter speed enough to 1/60 sec would be to use a ND filter. So the all important question, which ND filter do I need?
Well that really depends on the conditions you are shooting in. Living in Southern California I shoot in bright sunlight probably 90% of the time, and the filter that works the best for me is the ND32 which allows me to get the shutter speed very close to 1/60 sec. There are two companies that I'm familiar with that sell the ND32, which are Taco and Polar Pro. To go along with the ND32 I also have a ND16 to use when it is slightly cloudy, and with those two filters I am able to shoot under most SoCal conditions successfully. ND2, ND4, or ND8 filter would only work in cloudier and darker situations and a ND64 filter would work in extremely bright situations, such as in the desert or in snow.
In order for the best looking footage, it is important to shoot video in D-Log on the phantom 3/4. In D-Log even if your footage is slightly over/under exposed it can be corrected later in post editing. Another thing, if you cannot perfectly achieve a shutter speed that is exactly double the frame rate (fps) just shoot as close as possible. One last point to touch on is that ND filters can be used to slow down the shutter speed enough to create movement in photographs. Examples would include showing a car/train in motion or a stream in motion.