You are welcome. By the way, there are also situations you need ND, or PL ND filters when shooting aerial images. I will demonstrate what I mean with one example here.
View attachment 161315
The image above was shot for a client in a cold winter morning. There are three issues I need to resolve, when planning this shot.
1) The clouds in the sky and the snow on the ground created lots of light reflection and refractions on the river surface and leaves of the trees. I need to find a way to get rid of that.
2) The sky is overly bright compared to the waterfall hiding in the shadow. I need a way to balance the light between sky and waterfall, and make the sky "pop".
3) I need a slower shutter speed to create the "silky smooth" effect for the waterfall. A maximum shutter speed of 1/15 sec is needed to create such effect. 1-2 sec would be ideal.
PL filter can resolve issue 1) and 2).
ND filter can resolve issue 3). If you don't have ND filter, PL filter also helps to certain degree, since it cuts down the light input by 1.5-2 stops.
ND-PL filter resolves all the issues above. This is the ideal filter I need to achieve the final result.
Because I did not have ND-PL filter at the moment of the shot, I used PL filter instead. The image was shot at 1/15 sec and F/11. You will notice certain amount of "silky smooth" effect for the waterfall, but not particular strong. If I had ND-PL filter at that moment of time, I could have shot the image with 1-2 sec shutter speed, and achieve even better final result.