I am a total noob when it comes to photography so wish to tap into the unparralled knowledge of those in the know . I have bought ND4 and ND8 filters for my P3S, but have no idea what they really do other than reduce the amount of light entering the lens. I know they can help in harsh lighting conditions, but don't know the mechanics of how. I have read of them changing the  'f stops', but again have no clue what these are either. Would some kind person, who knows what they are talking about and can put it into language a thicko can understand, please explain what settings do I need to change when the ND4 or ND8 filter is fitted, or is the camera on the P3S smart enough that I can just leave it in 'Auto' and swap over the filters as I need to.

I am no photography expert, but I do know a bit and use ND filters myself.  You are spot on in that they simply reduce the amount of light hitting the sensor.  ND stands for "Neutral Density" and refers to the fact that it reduces all wavelengths equally, as opposed to only certain colours or UV etc.  The ND4 and ND8 numbers mean how many "stops" it reduces it by, but basically just means ND8 will block more light than the ND4 filter.  On a normal camera you would change the aperture, but Phantom cameras have a fixed aperture, which is why you need to use ND filters if there is too much light.

One common use for ND filters is reducing the so called "jello" effect, where vibration is seen in the video.  It does this because as the light entering the camera reduces, the camera is forced to lower the shutter speed to get the same exposure, and this generates some motion blur as the light hits the sensor over a longer period of time.  This motion blur helps to mask the effects of the vibration.

You can leave the settings on auto if you want, and the Phantom will adjust the settings automatically to get the right exposure.  If you switch to manual settings, you would need to reduce the shutter speed to brighten up the image to compensate for the lack of light the filter causes.

Thanks probonic, at least now I know it is the shutter speed I need to 'play about' with if needed. I think it will just be a series of short flights of the same image at the same time of day (ish) making notes and changing the filters and then looking at the video post flying to see what are the best settings.

Use ND8 in brightest conditions, and ND4 in cloudy or low light conditions. They let you go into the manual settings and lower the shutter speed, lower the shutter speed as much as you can without over exposing the image and you will have smooth video.