Most people who take pictures are only concerned about shutter speed. A fast shutter speed helps the sharpness of an image due to movement. A fast shutter speed can “stop” action. Consider the following two images:
The slower shutter speed shows the water, pretty much like I saw it. But the faster shutter speed of the one on the right freezes the water in mid-fall; something you can’t distinguish with the naked-eye. (You can click the images to see them larger). Which you prefer is a creative choice; you get a sense of action on the left; on the right you get a surprising image, but no sense of movement.
Okay, but I want to point out another big difference that many people overlook. Both pictures were taken, focusing on the same point in the water. Look at the water and the edge of the pool, in the background; that wall is about 40 yards away from the waterfall. The small aperture (ƒ⁄32) has a very wide depth-of-field; ƒ⁄5.6, on the other hand, has a very shallow dept-of -field, so the water remains in focus while the wall is blurred out. Which is better is another creative choice; everything in focus (aside from the movement of the water); or separation of the foreground subject from the background, on the right.
Note that the creative choice of the shutter speed to “control” the movement has a direct impact on how distinct the subject is from its background.
It might have been possible to adjust the results to freeze the water and keep the background in focus (or blurry water with blurry background) by also adjusting the ISO sensitivity of the sensor (which would impact the noise of the image).
I’ll be writing more about the balance of these settings in another post (once I dig up some examples).