To blur or not to blur; The effect of shutter speed and lens focal length
Two basic waterfall photo effects:
- Slow shutter speeds blur the motion producing yielding a soft dreamy effect. However, extremely slow shutter speeds may produce a whiteout and become ineffective.
- Fast shutter speeds freeze the motion. On breezy days the water will appear to be falling in sheets, and that will convey power as opposed the softness of motion blur.
Imagine taking a picture of Niagara Falls or another high volume waterfall. If you blur the motion in a photo, you will render the falls into a total whiteout (or 12-13% gray-out if you do not compensate the exposure). There will be no texture in the falling water. The solution is to freeze the motion with a fast shutter speed which will also convey the great power of the waterfall.
Motion blur in waterfalls is a matter of how far each drop of water falls through the frame while the camera's shutter is open. Shutter speed and lens focal length are both factors. Try photographing a waterfall using a wide angle and telephoto lens from the same viewing position. Use the same shutter speed with both lenses. The two photos will have radically different amounts of motion blur. In each case the water droplets fell the same distance, but the field of view in the two lenses are different. A droplet may fall across the entire frame in the telephoto photo, but perhaps only a small portion of the frame with the wide angle.
Your distance from the waterfall is also a factor, although this may not be too relevant for hiking photography. Our position is usually dictated by the presence of trees and other obstacles. By necessity we are generally very close in order to get a clear view of the waterfall. However, if you could move further from or closer to the waterfall you would change the field of view of your photo in a manner similar to using different lens focal lengths.
Some guidelines I developed while using a 35mm film camera. For point and shoot and cropped-view dSLR cameras you would need to convert your lens focal lengths to equivalent 35mm camera focal lengths using your crop factor multiplier. Or more simply; just use the suggested shutter speeds for the various lens types (wide angle, telephoto, etc.).
For motion blur use a shutter speed that is 3-4 stops slower than 1/lens_focal_length. For example:
1/2 - 1 second with a 17mm lens (ultra wide angle)
1/4 - 1/2 second with a 24mm lens (wide angle)
1/8 - 1/4 second with a 50mm lens (normal concentrated view)
1/30 - 1/15 second with a 200mm lens (telephoto)
The method to freeze water motion is the converse of the blur method. Use a shutter speed that is 3-4 stops faster than 1/lens_focal_length. For example:
1/125 - 1/250 second with a 17mm lens (ultra wide angle)
1/250 - 1/500 second with a 24mm lens (wide angle)
1/500 - 1/1000 second with a 50mm lens (normal concentrated view)
1/2000 - 1/4000 second with a 200mm lens (telephoto)
For completeness: I find that shutter speeds close to 1/lens_focal_length produce a particularly unattractive amount of motion blur. The streaks appear short and staccato. This is much less pleasing than either the blur or freeze effects above.
YMMV, do some experimentation to find what pleases you. Personal preference comes into play. It is always a good idea to take multiple photos in the field, perhaps bracketing both shutter speed and exposure. You can then select the most pleasing from the multiple images. Note: exposure can be easily altered in post processing while the motion blur cannot.
I do apologize for the length of this dissertation, and yes it would be better with some photo examples.