To blur falling water, you need a slow shutter speed. The question is, though, how slow? For example, I used ¼ second for this portion of Epupa Falls in northern Namibia and low how soft the water is. I used a 100-400mm telephoto for this image set to about 300mm, but had I used a wide angle lens with the same shutter speed, the water wouldn’t look as blurred. In determining the speed of the shutter for falling water, the factors that affect that choice are: how fast the water is flowing, what focal length you’re using, and how much abstraction you want in the water. For wide angle shots of waterfalls or rivers moving fast, I recommend at least one full second. When using a telephoto lens, ¼ second and slower is good. Obviously you need a tripod, too, and the ideal light is overcast. In order to use such slow shutter speeds, you will need to close the lens all the way down to f/22 or f/32, and the ISO has to be low, too. For this picture, I used f/32 and 100 ISO. The small lens aperture was important because it was absolutely necessary to have complete depth of field. The foreground portion of the landscape had to be as sharp as the background. Even though the water is blurred due to motion, it's still in focus and the surrounding rocks tack sharp.
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