What’s the effect? Producing the ‘silky water’ effect in a waterfall shot can be easy to achieve, but it depends on a number of factors: the volume of water, the camera angle, and the type of waterfall all have a bearing on the shutter speed used.
There has long been a debate on whether it’s better to use a long or short exposure to photograph waterfalls. A short exposure will render the water looking frozen, giving the impression of tremendous power, while a longer exposure of at least 1 sec will turn it silky smooth for a dreamy look. Some people may dislike this ‘milky’ texture, but it does come down to personal preference.
If there is a lot of water spilling over the falls, use a slightly shorter exposure, otherwise it could become just a featureless white veil. This is especially true if your camera angle is straight on to the falls. Consider also the type of waterfall you’re shooting. I prefer ones with lots of ledges for the water to cascade over.
What’s the time? There isn’t one exposure time that works for all waterfalls, as a number of factors have to be considered. However, you will definitely need a tripod for this technique.
Many waterfalls are situated in the depths of forests. This is good, as dark conditions naturally require a longer shutter speed, and direct sunlight can result in very high-contrast images. Overcast conditions are the best for shooting forest waterfalls.
I always use a polarising filter to cut the reflections from the foliage and the highlights on the water, but this also cuts one to two stops of light going through the lens, so requires a longer shutter speed.
The aperture you use will have a bearing on the shutter speed. You could shoot at a small aperture such as f/16 or f/22 to achieve a long enough exposure, and this is great if you also need more depth of field because you are including something in the foreground, but it is not ideal for getting the sharpest possible images. I try to use my optimal aperture of f/8 with my Nikon 24- 70mm lens whenever possible. In order to shoot at f/8 and still get a long enough exposure, I use the Lee Little Stopper Neutral Density filter. This blocks out six stops of light, permitting long exposures even in bright daylight conditions.
I shot this image of Iguazu Falls at f/8 and a shutter speed of 3 secs using a polarising filter plus the Lee Little Stopper. Compare it with the inset image of the falls, shot at 1/160 sec; as you can see, the inset image doesn’t have the same appeal.