GEAR
The technique to soften water is in controlling the shutter speed and the most important item aside from your camera and lens is a sturdy tripod. You may also want to bring a polarizer and a neutral density filter, especially for daytime shots. A remote shutter trigger is a convenient tool and a back-saving device when your camera is positioned low to the ground. Finally, a rain cover for your camera is good idea when shooting by water — especially near waterfalls or breaking waves.
HOW LONG IS “LONG”?
Sure, we know the trick to softening water is in a slow shutter speed and long exposure but what exactly do we mean by “long”? That answer really depends on a number of variables, but as a rule of thumb, any shutter speed that is too slow for a hand-held in-focus shot can be considered a long exposure. I can see some of you rolling your eyes — “just give me a number!” — so for the sake of simplicity let’s say 1/15 of a second up to 1 second is a medium length exposure, and anything over 1 second is “long”.
“That’s quite a range” (another eye roll) — it sure is, and there’s a certain amount of experimentation needed to arrive at the optimal shutter speed for a given shot. It’s also the case that two exposures of the same scene can look quite different even at the same shutter speed.
TECHNIQUE
A good tripod is essential to pulling this off, so ensure your camera is mounted and stable before you begin. Put your camera in manual mode and set the aperture to a narrow f-stop such as f/16 or f/22. Set your ISO to its lowest native setting (100 for most Canon cameras; 64, 100, or 160 for different Nikon bodies). Evaluate how fast the water is moving relative to the camera and dial in a shutter speed of a few seconds to start. Look at your exposure meter — if the scene is underexposed, you can open the aperture a bit wider or increase the ISO. If the scene is overexposed, shorten the shutter time a bit. In bright daytime scenes it may be necessary to use a neutral density filter in order to achieve the desired shutter speed. Adjust the shutter speed until the water is softened to taste. Take several exposures and choose the best one.