I was talking to an older photographer the other day. He was from the film days. The days, when everything had to be done right in camera. And, he asked me if I used filters. In film photography, filters were a very large part of the process. You used them to color balance film, or lights, and used them for special effects and all sorts of things.
Digital Photography has changed that, and, on the whole, we do not use filters very often, because we can achieve the same or similar look that filters provided.
There are still a few filters that I use almost every time that I go out. One of my favorite filters is the B&W ND110. The ND110 filter is a 10 stop filter, meaning that it reduces your exposure by 10 stops. So if you are shooting a scene at 1/8000 of a second and then you add this filter you will need to change your speed to 1/8th of a second. It is a great filter any time you are shooting images of water, as the lower shutter speeds allow the water to show movement. Here is an example:
You can see that both images are basically the same, the only thing that really changes is that the shutter is left open long enough for all the movement in the water is smoothed out and the clouds overhead are beginning to blur.
ND filters come in many flavors, you can buy them from 1 stop all the way to 20 stop. The 10 stop filter that I used on the above images, appears completely opaque. You cannot see through it at all. Your camera will not be able to focus or meter accurately with the filter in place, and you will not be able to see through the viewfinder with it in place. You frame up your shot, meter, then focus and then lock all this down. Calculate the 10 stop difference ( or use a handy little app on your iphone like PHOforPHO) then take the pic. It is pretty easy to get 5-10 minute exposures on days when you have heavy cloud cover, in bright sunlight 30-45 seconds exposures are pretty straight forward.