Go with the flow
What’s the effect? When you want to capture the drama of a cloud formation by freezing the clouds, yet have enough movement in the foreground to ensure, say, a river appears to be flowing rather than completely frozen, you have to carefully consider your shutter speed. If you use too long an exposure, the clouds will start to blur and this can be unsettling to anyone looking at your picture. Clouds look good with either a short enough exposure to freeze them or a really long exposure to get lots of movement (see page 35); anything in between can begin to look odd. Too long an exposure would also run the risk of losing texture in the water and burning out highlights.
What’s the time? The optimal shutter speed of 1/5 sec was the perfect compromise here, enabling me to achieve a flowing river with texture in the water and yet freeze the clouds and show their massive, billowing shape. I wanted to capture the swiftly moving clouds over the Cuillins on the Isle of Skye before they drifted out of the scene. Even so, timing was as important as it is when capturing waves. I waited until they were over the mountains to give the impression of an erupting volcano. I used the diagonal line of the flowing river to reinforce the angular composition. As there was only a little water flowing in the river, a shutter speed of 1/5 sec was slow enough to get some motion without losing highlight detail.