No kids in sight on this busy playground - 18mm; 50 sec; f/13; ISO 50
The picture above was taken at a playground we usually go to. I wanted to take a picture of one of the play areas but, since the place was full of running kids, some of them would end up inevitably on the picture. By attaching a 10-Stop Neutral Density (ND) filter to my lens, I was able to increase the shutter speed to 50 seconds, even tough it was very bright on that location. Thanks to the very long shutter speed, any running kid that passed in front of the camera was not bright enough to register permanently on the final image.
The ND filter used was the Heliopan 82mm 3.0 ND filter. I also used the 82mm polarizer filter, also from Heliopan, which gave me another 1 to 1.5-stops, to compound the effect. The polarizer filter was screwed on a Canon 16-35mm f/2.8 lens, and then the ND filter was screwed on the polarizer filter. I thought these 2 filters stacked on a wide angle lens like the Canon 16-35mm would cause some strong vignetting, but that turned out to be minimal. Just cropping a little bit of the picture fixed the problem. The camera was on a tripod, of course. Notice that I also used the lowest possible ISO in my camera (ISO 50) to "drag" the shutter speed even more.
The only annoyance of using the 10-Stop ND filter is that, since it is so dark, the camera auto focus will likely not work and the view finder goes totally black. You have to focus before attaching the filter on the lens.
Besides making the playground empty, the slow shutter speed helped create a very interesting effect on the sky thanks to the moving bright clouds. Very cool.