Sony a7Rii & 16-35mm f/4.0 (@ 21mm)
30 seconds | f/11.0 | ISO 50
A 1/5 of a second exposure will show the movement of the water whereas a 10-15 second exposure will create a smoother water flow. Whatever your preference you will most likely need a 'neutral density (ND) filter' - this will help cut the light, enabling you to create longer exposures.
If you don't have an ND filter you can always try (on aperture priority mode) adjusting your ISO to the lowest possible (ISO 50 on my camera) and increasing the aperture (to reduce the light) to something like f/11 or f/14 to try and achieve a longer exposure. I would avoid using too high an aperture as you will start to create light diffraction and lose sharpness in the image.
Getting the right exposure
You are often trying to capture very bright water surrounded by very dark surroundings. If left on auto your camera will expose incorrectly; it will try to expose for the shadows in the scene and probably over expose to the point the falls/water is blown out and not recoverable in post. I often find I need to be at least 1-stop under to ensure the water is not blown out. This will mean the overall image is very dark but the shadows can be recovered in post processing.
Check out the histogram [RAW vs. Processed] - the highlights are not blown out and the shadows have been recovered in the processed image.
Not essential but help to cut out the glare from the water reflections and rocks in the scene. These filters can be particularly useful on bright days.
As a guideline you can see the processing used for the example image. To make the waterfall pop you should bring up the whites, however this makes the scene quite saturated so I often bring down the vibrancy/saturation a little, too.
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