The experience of interesting landscapes is often shaped not only by the presence of water, but also by the way it moves. A raging torrent looks and feels very different from a placid, meandering stream, and the character of the ocean can change dramatically within a few hours, from warm and enticing to threatening and malevolent. How can photographers capture these moods?
Image Blurring Effects
First, think about what you want to achieve. Broadly, faster shutter speeds give a greater sense of realism, are better for more dramatic effects and are often a safer bet for an acceptable shot. Slow speeds will blur the movement with effects that can be pleasingly atmospheric but which sometimes appear fake and irritating.
Whether the goal is to increase blur or decrease it, the key is to know how to change the shutter speed without compromising the exposure. Digital photographers have three kinds of controls to work with.
Controls on Shutter Speed
There may be some scope for changing the aperture, but probably not much. Aperture affects the depth of field, so for many shots the aperture value is best left unchanged once the composition is set.
With the same aperture, the ISO value can have a big effect on shutter speed. For example at f22 a shutter speed of a tenth of a second at ISO 800 should give the same exposure value as 1.6 seconds at an ISO of 50, producing very different levels of blurring. But beware of noise creeping in at high ISO values and in dark parts of the frame.
A neutral density or polarizing filter can be effective in relatively bright conditions where the aim is to slow the shot down and create blur. At ISO 100 for example, a polarizing filter can change a shutter speed of an eighth to more than a second, again producing quite a different kind of picture.
High Dynamic Range Techniques
The standard approach to creating high dynamic range photographs is to take three or more otherwise identical shots in succession with different exposure values, and use software to merge the resulting images. Many cameras provide automatic exposure bracketing functions so that the shots can be taken quickly; even so, there will be a lag between each exposure and any moving water will be blurred when the images are merged. The amount of blur is difficult or impossible to control.
A way around the problem is to set up a shot using the above controls, and to use the resulting single RAW file to create a tiff image from each of three or more different exposures: one at the standard exposure value and one each a stop above and below the standard. Opinions vary on whether this approach counts as high dynamic range photography, but it certainly allows a higher dynamic range than can be achieved within a single jpeg image while controlling the amount of blur.
Systematic Experimentation
As with many kinds of creative work, there is no formula that will work in every situation. Photographers learn by systematically experimenting with different settings and taking a careful look at the results. With tripod set, try several combinations of aperture, ISO, filters and shutter speeds within the same composition.
Chances are, there will be a fine shot among the results. Before sending it off to your on-line or local gallery make sure you understand why certain combinations of settings worked while others failed, and chalk up another lesson learned.
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